Exposure to trace elements and risk of skin cancer: A systematic review of epidemiologic studies.

Abstract

Exposure to environmental trace elements has been studied in relation to many cancers. However, an association between exposure to trace elements and skin cancer remains less understood. Therefore, we conducted a systematic review of published epidemiologic literature examining the association between exposure to trace elements, and risk of melanoma and keratinocyte carcinoma in humans. We identified epidemiologic studies investigating exposure to arsenic, cadmium, chromium, copper, iron, selenium, and zinc and risk of skin cancer in humans. Among the minerals, arsenic, selenium, and zinc had more than 5 studies available. Exposure to arsenic was associated with increased risk of keratinocyte carcinoma, while too few studies existed on melanoma to draw conclusions. Exposure to selenium was associated with possible increased risk of keratinocyte carcinoma. Studies of zinc and skin cancer were case-control in design and were found to have inconsistent associations. The data on the association between cadmium, chromium, copper, and iron and risk of skin cancer remain too sparse to draw any conclusions. In summary, epidemiologic studies on exposure to trace elements and cutaneous malignancies are limited. Studies with larger sample sizes and prospective designs are warranted to improve our knowledge of trace elements and skin cancer.

Introduction

Keratinocyte carcinoma (KC), including cutaneous basal (BCC) and squamous cell carcinoma (SCC), are the most commonly diagnosed cancers in the United States (US) (1–3). BCCs account for nearly 80% of all KC diagnosed annually (3–5). The remaining 20% of KC cases are mostly SCC (3,6,7).

Melanoma is a malignant skin tumor that arises from melanocytes (8). Though melanoma accounts for less than 5% of all cutaneous malignancies, it is the most lethal, representing 75% of all deaths due to skin cancer (1,9). Overall, melanoma and KCs represent a significant economic and disease burden that is projected to continue to increase in the coming years (10).

Trace elements include metals that are widely distributed in the natural environment, as well as in numerous industrial, domestic, and agricultural settings. Concerns regarding exposure to potential health hazards from these metals have prompted extensive research on the subject of metal carcinogenicity (11–13). Chromium, for example, has been associated with increased lung cancer incidence (13). Similarly, arsenic has been linked to increased mortality from bladder and kidney cancers (11). The subject of metal carcinogenicity is one of increasing importance, as it represents a potentially modifiable risk factor. Exposure to trace elements has been implicated in the pathogenesis of skin cancers (11,14). However, with the exception of arsenic (11), the degree of association and potential underlying mechanisms are still poorly understood.

This review examines existing epidemiologic literature on trace elements and skin cancer risk. These elements include arsenic, cadmium, chromium, copper, iron, selenium, and zinc.

Materials and Methods

Search Strategy

We sought to identify epidemiologic studies relevant to the research question: Which environmental trace element exposures are associated with skin cancer? Searches were performed in PubMed, Web of Science, and Embase (1972 –July 2018) with the terms “melanoma” OR “squamous cell carcinoma” OR “basal cell carcinoma” OR “keratinocyte carcinoma” OR “non-melanoma skin cancer” OR “skin cancer,” AND with “metals” OR “trace metals” OR “heavy metals” OR “minerals” OR “environmental exposure” OR “occupational exposure.” From this search we were able to select metals with existing literature relating to skin cancer. A secondary search included the above terms with “arsenic” OR “cadmium” OR “chromium” OR “copper” OR “iron” OR “selenium” OR “zinc” (presented in Tables 1–5). We also searched aluminum, beryllium, calcium, cobalt, lead, manganese, magnesium, mercury, and nickel given prior published possible associations with other cancers or role in normal skin development, homeostasis, and repair (12,13,15–19).

Table 1.

Epidemiologic studies of arsenic exposure and (^A) cutaneous melanoma and (^B) keratinocyte carcinoma listed by study design and year.

Reference	Study Design	Demographics	Cases/ controls or total participants	Exposures	Results (RR/OR/HR and 95% confidence interval)	Covariate Adjustment
A. Melanoma
Cohort
Dennis et al., 2010 (53)	Prospective cohort study (1993–2005) Agricultural Health Study	Sex: NS Mean age: Cases: 57 Controls: 48 Country (Region): US (Iowa, North Carolina) Ethnicity: NS	150MM/24,704	Arsenic pesticide	OR = 1.3 (0.7–2.4) for never used vs. ever used	Age at enrollment, sex
Baastrup et al., 2008 (54)	Prospective cohort study	Sex: 26,876M/29,502F Median age: 56 Country (Region): Denmark (Copenhagen, Aarhus)	147MM/56,378	Level of arsenic in the drinking water by time-weighted average exposure and by cumulated exposure	RR: 0.80 (0.59–1.08) for time-weighted average exposure of arsenic (per ug/L) RR: 0.96 (0.89–1.04) for cumulated arsenic exposure (per 5 mg)	Education, skin reaction to sun, suntanned during summer, area of enrollment
Case-Control
Beane Freeman et al., 2004 (55)	Case control study (population-based)	Sex: M/F Cases: 205M/163F Controls: 240M/133F Median age: Cases: 60 Controls: 62 Country (Region): US (Iowa) Ethnicity: Caucasian	326MM/329 Controls diagnosed with colorectal cancer and frequency matched for sex and age	Toenail arsenic concentration	MM associated with highest quartile (≥0.084 ug/g) compared to lowest quartile (≤ 0.020 ug/g): OR = 2.1 (1.4–3.3) Ptrend 0.001	Age, sex, education
B. Keratinocyte Carcinoma
Cohort
Hsueh et al., 1997 (63)	Retrospective cohort study	Sex:468M/613F Age: ≥30 Country: Taiwan	26KC/497	Cumulative arsenic exposure	Cumulative arsenic exposure (mg/L-yr) OR = 2.82 (0.25–31.87) for 0.1–10.6 2.61 (0.30–22.90) for 10.7–17.7 7.58 (0.95–60.3) for ≥17.7	Age, sex, education level
Baastrup et al., 2008 (54)	Prospective cohort study	Sex: 26,876M/29,502F Median age: 56 Country (Region): Denmark (Copenhagen, Aarhus)	1,010 KC/56,378	Level of arsenic in the drinking water by time-weighted average exposure and by cumulated exposure	RR: 0.99 (0.94–1.06) for time weighted average exposure of arsenic (per ug/L) RR: 0.99 (0.97–1.01) for cumulated arsenic exposure (per 5 mg)	Adjusted: education, skin reaction to sun, suntanned during summer, occupation, area of enrollment
Case-Control
Yu et al., 2000 (74)	Case-control study (hospital-based)	Sex: 28M/24F Mean age: 63 Country (Region): Taiwan (Southwest region) Ethnicity: NS	2BCC, 19 Bowen’s diseases (SCC in situ), 6 hyperkeratosis/hyperpigmentation/26 Controls matched by age and sex	Urine levels of inorganic arsenic (InAs), methylarsonic acid (MMA) and dimethylarsinic acid (DMA)	Skin lesions associated with high % In As vs. low: OR=3.50 (0.73–16.85) high % MMA vs. low: OR = 5.50 (1.22–24.81) Low %DMA vs high: OR = 3.25 (1.06–9.97)	Sex, age, cigarette smoking, hepatitis B surface antigen, alcohol consumption, and regular tea intake.
Karagas et al., 2001 (76)	Case-control study (population-based)	Sex: BCC:182M/102F SCC: 388M/249F Controls: 315M/ 209F Age range: 25–74 Country (Region): US, New Hampshire Ethnicity: NS	587 BCC, 284 SCC/524 Controls matched by age and sex	Toenail arsenic concentration	Above the 97th percentile (≥0.345ug/g) compared to median (≤ 0.089 ug/g) SCC OR = 2.07 (0.92–4.66) BCC OR = 1.44 (0.74–2.81)	Age and sex
Chen et al., 2003 (75)	Case-control study (hospital-based; 1996–1999)	Sex: Case: 48M/28F Control: 131M/ 93F Age range: >30 Country (Region): Taiwan (Southwest region) Ethnicity: NS	76KC/224	Percentage of urinary arsenic species, arsenic methylation ability, and cumulative arsenic exposure	Mean cumulative arsenic exposure (mg/L-year): Cases: 15.33 ± 18.8 Controls: 8.14 ± 15.48 P = 0.002	Age, sex, body-mass index (BMI), cigarette smoking, the use of hair dye, and education
Rosales-Castillo et al, 2004 (81)	Case-control study (hospital-based)	Sex: Case: 71% male Control: 21% male Age: mean 63 for cases and 47 for controls Country (Region): Mexico Ethnicity: NS	42 KC/48	Historical arsenic exposure (arsenic concentration in the drinking water in the town of residency*years lived in the town/age)	Compared with low arsenic exposure and negative HPV seropositivity; OR=4.53 (0.63–32.76) for high arsenic exposure and negative HPV seropositivity OR=9.04 (1.48–55.41) for low arsenic exposure and positive HPV seropositivity OR=16.50 (2.97–91.75) for high arsenic exposure and positive HPV seropositivity	Age, gender, and sun exposure
Leonardi et al, 2012 (78)	Case-control study (hospital-based)	Sex: Case: 237M/292F Control: 278M/262 F Age range: >30 Country (Region): Hungary, Romania, and Slovakia Ethnicity: NS	529 BCC/540	Lifetime average inorganic arsenic (iAs) concentration in residential drinking water, peak daily dose rate, cumulative iAs dose	OR=3.03 (1.70–5.41) for 19.5–167.3 vs. <0.68 lifetime average iAs concentration (ug/L) Ptrend 0.001 OR=2.50 (1.39–4.49) for 32.2–242.1 vs <0.73 peak daily iAs dose rate (ug/d) Ptrend 0.001 OR=2.63 (1.45–4.78) for 0.55–4.46 vs. <0.01 cumulative iAs dose (g) Ptrend 0.001	County, age, sex, education, skin response to 1-hr midday sun, and skin complexion.
Gilbert-Diamond et al., 2013 (77)	Case-control study (population-based)	Sex: SCC: 284M/186F Control: 258M/189F Age range: 25–74 Country (Region): US, New Hampshire Ethnicity: Caucasian	323 SCC/319 Controls matched by age, sex and state of residence	Urinary levels of InAs, MMA, and DMA, sum (ΣAs) of the species	For each ln-transformed μg/L increase: ln(ΣInAs) OR = 1.37 (1.04–1.80) ln(iAs) OR = 1.20 (0.97–1.49) ln(MMA) OR = 1.34 (1.04–1.71) ln(DMA) OR = 1.34 (1.03–1.74)	Age, sex, BMI, education, smoking status, skin reaction to chronic sun exposure, and urinary creatinine concentration
Surdu et al., 2013 (64)	Case-control study (hospital-based)	Sex: BCC: 231M/284F SCC: 38M/32F Controls: 272M/255F Age range 30–79 Countries (Region): Hungary, Romania, Slovakia Ethnicity: NS	515 BCC, 70 SCC/527 controls matched to by county of residence, sex and 5-year-age group	Cumulative lifetime workplace dust/fume arsenic exposure (>232.5hr)	≥7,232.5hr vs ≤105hr KC: OR = 1.94 (0.76–4.95) BCC: OR = 1.90 (0.72–4.99) SCC: OR = 2.69 (0.50–14.59)	Sex, age, county of residence, family history of cancer, skin propensity to sunburns and lifetime average arsenic concentration in drinking water
Cross-sectional
Haupert et al., 1996 (80)	Cross-sectional study	Sex: 727M/731F Age: All ages Country (Region): US (Outagamie and Winnebago counties, Wisconsin) Ethnicity: NS	Overall KC 17/1,836	Drinking water arsenic consumption	>50 μg/day compared to <5 μg/day: Overall KC RR = 3.28 (2.17–4.40)	Age and sex

Age given in years; RR = relative risk; HR = hazard ratio; OR = odds ratio; CI = confidence Interval; NS=Not specified

^A.MM = cutaneous malignant melanoma

^B.KC = Keratinocyte carcinoma; SCC = cutaneous squamous cell carcinoma; BCC = cutaneous basal cell carcinoma

Table 5.

Summary of number of epidemiologic studies of metals and skin cancer^*

Metal	Type of skin cancer	Number of case-control studies	Number of cohort studies and nested case-control studies	Number of clinical trials
Arsenic	Melanoma	1	2	0
	KC	7	2 (one retrospective and one prospective cohort)	0
Cadmium/Chromium	Melanoma	1	1 (cadmium only and among melanoma cases)	0
	KC	0	0	0
Iron	Melanoma	3	0	0
	KC	0	0	0
Copper	Melanoma	2	0	0
	KC	2	0	0
Zinc	Melanoma	6	0	0
	KC	1	0	0
Selenium	Melanoma	3	3	1
	KC	2	5	2

^*The numbers are based on number of studies, not number of publications. Some studies had multiple publications.

Arsenic and KC also had one cross-sectional study.

Study Selection

Studies reviewed reported exposure to one of the above-mentioned minerals in relation to risk of skin cancer in adult populations. All selected articles were original research, peer-reviewed, published in English, and specifically evaluated exposure to metal directly. If the full text of articles were unavailable, they were acknowledged in the text, but excluded in the tables. Only human epidemiologic studies were included. For example, nickel was excluded from the review since we found only non-human studies investigating nickel exposure and skin cancer (20–22). Only studies that explicitly investigated exposure to the mineral itself were included. For example, mercury was excluded from this review since it only has been studied indirectly with regard to occupations with possible exposure and risk of melanoma (23,24). Only minerals with literature suggesting a possible biological mechanism for risk of skin cancer were included. For example, lead was excluded given that there was no experimental data to suggest risk. Only one epidemiological study was found about lead exposure and skin cancer, and it was a case-control study examining toenail lead levels and melanoma risk, which reported no association (25). We found no studies on exposure to aluminum, beryllium, calcium, cobalt, manganese, and magnesium and risk of skin cancer. Thus, we excluded these elements. The elements we ultimately evaluated were arsenic, cadmium, chromium, copper, iron, selenium, and zinc.

Non-epidemiologic studies, including non-human experiments, were discussed in the text to supplement discussion of cancer risk. We included randomized controlled trials (RCTs), cohort, case-control, and cross-sectional studies. Ecological studies were discussed in text, but excluded from tables given the diminished quality of design with risk of data inaccuracy and difficulty to control for potential confounders among other limitations (26). Furthermore, ecological studies often investigated exposure to metals indirectly. However, some ecological studies were described in the text to help evaluate the totality of evidence. Details about study design, study population, exposure source, exposure measures, and results were recorded.

Included studies are shown in Tables 1–4, and briefly discussed in the text. For arsenic, zinc, and selenium, which had more than 5 studies available, flow charts of available studies were provided in supplementary material (Supplementary Figures 1–3). Based on quality of study design, more emphasis was placed on RCTs, followed by cohort studies, then case-control and cross-sectional studies, as the later studies are increasingly more prone to bias (26). This was also the order of discussion of the studies in text, and the order of listed studies in Tables 1–4. When multiple publications were available from the same population, we used the most recent publications and excluded earlier ones (25,27,28).

Table 4.

Epidemiologic studies of selenium and (^A) cutaneous melanoma and (^B) keratinocyte carcinoma

Selenium
Reference	Study Design	Demographics	Cases/controls or total participants	Exposures	Results (RR/OR/HR and 95% confidence interval)	Covariate Adjustment
A. Melanoma
Randomized controlled trial
Duffield-Lillico et al., 2002 (235)	Double-blind, placebo-controlled randomized trial with KC history (1983–1996) The Nutritional Prevention of Cancer Trial	Sex: M/F Mean age: 63 Country (Region): US (East) Ethnicity: NS	11MM/621 in supplement group and 9MM/629 in placebo group	Supplementation of 200μg/d of Se versus placebo	RR = 1.21 (0.46–3.30) HR = 1.18 (0.49–2.85)	RR was unadjusted, HR was adjusted for sex, age, and smoking status
Nested Case-Control & Cohort
Garland et al., 1995 (209)	Nested case-control study (1976–1982)	Sex: F Age: 30–55 Country (Region): 11 States within the US Ethnicity: NS	63MM/63 controls were matched by year of birth and month of toenail return	Toenail selenium concentration	Highest tertile compared to lowest tertile of selenium exposure: OR = 1.66 (0.71–3.85)	Smoking status
Vinceti et al., 1998 (213)	Prospective cohort study (1975–1985)	Sex: 1021M/1044F Age: > 5 Country (Region): Italy (Reggio Emilia) Ethnicity: NS	8MM/2,065 exposed	Exposure to high levels of inorganic selenium in tap water	Standardized morbidity ratio: Male = 5.0 (1.6–12.0) Female = 3.2 (1.0–7.7)
Asgari et al., 2009 (210)	Prospective cohort study (2000–2006)	Sex: M/F Age range: 50–76 Country (Region): US (Washington State) Ethnicity: Caucasian	461/69,671	Supplemental selenium use over 10 years	≥50 μg/day Se compared to none: RR = 0.98 (0.69–1.41)	Age, sex, education, family history of melanoma, personal history of KC, history of mole removal, freckles between ages 10 and 20 years, ≥3 severe sunburns between ages 10 and 20 years, natural red or blond hair, reaction to 1 hour in strong sunlight
Case-control
Breslow et al., 1995 (212)	Case-control study (population-based) 000000000000000ppppppp77777	Sex: 55M/44F Age: >18 Country (Region): US (Washington county, Maryland) Ethnicity: Caucasian	30MM/60 Controls matched by age, sex	Serum selenium level	Highest tertile compared to lowest tertile of selenium exposure: OR = 0.9 (0.3–2.5)	Smoking, education, and hours between last meal and blood donation did not change results
Vinceti et al., 2012 (214)	Case-control study (population-based)	Sex: 26M/28F Age: 25 – 79 Country (Region): Italy (Modena province) Ethnicity: NS	54MM/56 Controls selected from regional population and matched for sex and age	Toenail, plasma, and dietary selenium concentration	Toenail (≥ 73 μg/g): OR: 0.77 (0.26–2.28) Ptrend = 0.633 Plasma (≥ 105 μg/l): OR: 6.42 (1.94–21.24) Ptrend = 0.002 Dietary (≥ 71 μg/d): OR: 0.59 (0.19–1.83) Ptrend = 0.34	Age, sex
Le Marchand et al., 2006 (211)	Case-control study (population-based)	Sex: M/F Mean age: Cases: 53 Controls: 52 Country (Region): US (Oahu, Hawaii) Ethnicity: Caucasian	278MM/278 Controls matched by age, ethnicity and sex	Plasma, erythrocyte and toenail concentrations of selenium	Plasma(μg/ml): ≥ 0.14 vs ≤0.12 Males: OR = 1.2 (0.7–2.2) (Ptrend = 0.53) Females: OR = 0.8 (0.4–1.6) Ptrend = 0.49) Erythrocyte (μg): ≥ 0.15 vs ≤0.12 Males: OR = 0.4 (0.4–14) (Ptrend = 0.40) for males; Females: OR = 1.0 (0.5–2.0) (Ptrend = 0.90) Toenail (μg/g): ≥1. 0 vs ≤0.86 Males: OR=0.9 (0.5–1.6) (Ptrend = 0.61) Females: OR = 1.0 (0.4–2.1) (Ptrend = 0.99)	Height, education, hair color, number of blistering sunburns at ages 10–17 years, ability to tan and lifetime ethanol intake. selenium-containing shampoo was additionally adjusted for toenail Se
B. Keratinocyte Carcinoma
Randomized controlled trial
Duffield-Lillico et al., 2003 (28)	Double-blind, placebo-controlled randomized clinical trial with history of KC (1983–1996) The Nutritional Prevention of Cancer Trial	Sex: M/F Median age: 65 Country (Region): Eastern US Ethnicity: NS	621 in selenium and 629 in placebo group	Supplementation of 200 μg/day of selenium versus placebo	By baseline Se concentrations: ≤ 105 μg/mL: 0.87 (0.02–1.22) 106–122: 1.49 (1.05–2.12) ≥122: 1.59 (1.11–2.30) BCC RR = 1.17 (1.02–1.35) HR = 1.09 (0.94–1.26) SCC RR = 1.32 (1.09–1.60) HR = 1.25 (1.03–1.51)	RRs are unadjusted, HRs are adjusted for sex, age, smoking status, clinic site, plasma selenium concentration, clinical sun damage, sunscreen use at baseline, and number of previous BCCs, SCCs, or total NMSCs in the 12 months before randomization
Dreno, 2007(218)	Placebo-controlled randomized trial with recent organ transplant recipients (2 years)	Sex: 127 M/57 F Median age: 44 Country (Region): France Ethnicity: 89% Caucasians	6/91 in selenium and 2/93 in placebo group	Supplementation of 200 μg/day of selenium versus placebo	OR=3.08, p=0.15
Reid et al., 2008 (217)	Double-blind, placebo-controlled randomized trial with KC history (1983–1993). Sub-study of the Nutritional Prevention of Cancer Trial (1989–1996)	Sex: M/F Mean age:64 Country (Region): US (Georgia) Ethnicity: NS	98KC/210 in Se group 108 KC/213 in placebo group	Selenium supplementation with 400 μg/day or 200 μg/day selenized yeast versus placebo	400 μg/day: Overall KC: HR = 0.91 (0.69–1.20) BCC: HR = 0.95 (0.69–1.29) SCC: HR = 1.05 (0.72–1.53) 200 μg/day: Overall KC: HR = 1.5 (1.13–2.04), Ptrend = 0.006 BCC: HR = 1.22 (0.88–1.70) SCC: HR = 1.88 (1.28–2.79), Ptrend = 0.001	Age, smoking status, sex
Nested Case-Control & Cohort
Knekt et al., 1990 (221)	Nested case-control study	Sex: M/F Age range: 15–99 Country (Region): Finland Ethnicity: NS	126 BCC/252 Controls matched by age, sex and municipality	Serum selenium levels	Highest quintile compared to lowest selenium levels: Males: RR = 0.86 (0.35–2.12) Females: RR = 1.54 (0.64–3.73)	Smoking status, occupation, BMI, parity, cholesterol, hematocrit
Breslow et al., 1995 (212)	Nested case-control study (population-based)	Sex: M/F Age: >18 Country (Region): Maryland, US Ethnicity: Caucasian	32/64 for BCC, 37/74 for SCC Controls matched by age and sex	Serum selenium levels	Highest tertile compared lowest selenium levels: BCC: 0.8 (0.1–4.5) SCC: 0.6 (0.2–1.5)	Smoking, education, and the hours between last meal and blood donation did not change the results
Karagas et al., 1997 (224)	Nested case-control study in a clinical trial of those with history of KC Skin Cancer Prevention Trial	Sex: 89%M/11%F Mean age: 67 Country (Region): US (New Hampshire, Minnesota, California) Ethnicity: NS	132 SCC/ 246 controls Controls were chosen at random and matched by age, sex, and study center	Plasma selenium levels	For the highest quartile versus lowest quartile selenium levels and SCC: OR =0.86 (0.47–1.58)
Davies et al., 2002 (219)	Nested case-control study EPIC-Norfolk Study	Sex: M/F Median age: M: 67 F: 65 Country (Region): Britain (Norfolk) Ethnicity: NS	14 SCC, 109 BCC/247 controls	Dietary selenium intake	For each 20 ug Se intake and overall KC: RR =1.07 (0.86–1.34)	Adjusted for body mass index and red hair
McNaughton et al., 2005 (222)	Nested case-control study The Nambour Skin Cancer Study	Sex: Cases: 39M/51F Controls: 39M/51F Mean age: 55 Country (Region): Australia (Nambour) Ethnicity: NS	90 BCC/90 Controls matched for age and sex	Dietary selenium intake and serum selenium levels	Highest quartile compared to lowest selenium intake and BCC: Dietary intake: OR = 1.13 (0.47–2.74) Serum level: OR = 0.86 (0.38–1.96)	Age, sex and self-prescribed supplement use
Heinen et al., 2007 (220)	Prospective cohort study (1996–2004) Nambour Skin Cancer Study	Sex: 454M/547F Median age: SCC: 65 BCC: 61 Country (Region): Australia (Nambour) Ethnicity: NS	116 SCC, 149 BCC /1001	Dietary selenium intake	For highest tertile compared to lowest tertile selenium intake: SCC: RR = 1.30 (0.77–2.30) BCC: RR = 0.95 (0.59–1.5)	Age, sex, energy intake, skin color, elastosis of the neck, smoking, treatment allocation, use of dietary supplements, history of skin cancer before 1996
Van der Pols et al., 2009 (223)	Sub-set of prospective cohort study (1996–2004) Nambour Skin Cancer Study	Sex: 223M/262F Mean age: SCC: 63 BCC: 61 Controls: 54 Country (Region): Australia (Queensland) Ethnicity: NS	77 BCC, 59 SCC/485	Serum selenium concentration	For highest tertile compared to lowest tertile: BCC: RR = 0.58 (0.32 – 1.07) SCC: RR = 0.49 (0.24 – 0.99)	Age, sex, pack-years of smoking, alcohol intake; time spent outdoors on weekdays, and history of skin cancer before 1996
Case-control
Clark., 1984 (236)	Case control study (hospital-based)	Sex: M/F Age: <76 years Country (Region): US (Wilson, North Carolina) Ethnicity: NS	142 BCC, 48 SCC, 50 BCC+SCC/ 103	Plasma selenium levels	High vs low selenium levels and overall KC: OR = 2.11 (1.25–3.56)	Age and sun damage
Sahl et al., 1995 (158)	Case-control study (hospital)	Sex: M/F Mean age: Cases: 65 Controls: 64 Country (Region): US (South Dakota) Ethnicity: NS	46 BCC/46 Controls matched by age, skin-type and sex	Mean daily selenium intake	BCC cases: 99 ± 6 (μg) Controls: 112 ± 6 (μg) P = 0.14

Age given in years; RR = relative risk; HR = hazard ratio; OR = odds ratio; CI = confidence Interval; NS=Not specified

^A.MM = cutaneous malignant melanoma

^B.KC = Keratinocyte carcinoma; SCC = cutaneous squamous cell carcinoma; BCC = cutaneous basal cell carcinoma

Arsenic

Arsenic is a metalloid found ubiquitously in soil, rocks, and water. Human exposure occurs from ingestion of arsenic contaminated water and foods including grain-based processed foods, dairy products, and fish (11,29–32). Daily intake of arsenic from food and beverages is generally in the range of 20–300 μg/d (11). Water pollution by arsenic is a worldwide problem with over 226 million persons exposed (33,34). Countries including Argentina, Bangladesh, Chile, India, Nepal, China, and Taiwan are reported to be among the most heavily affected by arsenic contamination (11,35,36).

Chronic exposure to arsenic has been associated with a variety of health problems including several types of cancer, neurological disease, cardiovascular disease, and perinatal conditions (37–42). Arsenic is considered a group “A” carcinogen by The US Environmental Protection Agency (EPA) and a group “I” carcinogen by the International Agency for Research on Cancer (IARC) that can cause cutaneous SCC, BCC, kidney, bladder, and lung tumors (11,43–45). The European Food Safety Authority (EFSA) determined that a dose between 0.3 – 8 μg/kg body weight/d is estimated to result in a 1 % increased risk of KC, lung, and bladder tumors (46).

Arsenic is also a co-carcinogen with ultraviolet radiation (UVR) (47,48), which can cause both keratinocyte and melanocyte damage (49–51). Compared to keratinocytes, melanocytes are more resistant to UVR-induced cytotoxicity. However, when keratinocytes or melanocytes are exposed to arsenite, which inhibits DNA repair through the enzyme poly ADP ribose polymerase 1, susceptibility to UVR damage becomes similarly enhanced in both cell types (52). The co-carcinogenic effects of arsenic and UVR could partly account for the epidemiologic findings suggesting an increased risk of melanoma and KC upon exposure to arsenic.

Melanoma

There are few studies that evaluate the association between arsenic exposure and melanoma (Table 1a). To our knowledge there are no RCTs that investigate arsenic exposure and risk of melanoma. A US cohort study found no association between exposure to arsenic-containing pesticides and melanoma (Table 1a) (53). Similarly, in a Danish cohort study, no association was found between exposure to arsenic in drinking water and risk for melanoma (Table 1a) (54). A US case-control study examined toenail arsenic exposure and melanoma using colorectal cancer patients as controls, and found an increased risk of melanoma with increasing toenail arsenic concentrations (Table 1a) (55). The association between arsenic exposure and melanoma risk needs to be evaluated in arsenic-endemic areas including Asian and Latin American countries (11,56). The effect of arsenic exposure on melanoma risk may be modified by genetic or constitutional factors, such as skin color and sun sensitivity, as Asian and Hispanic populations are more resistant than Caucasian populations to melanoma (52,57,58).

KC

The link between arsenic exposure and KC has been more extensively evaluated (11,43–45), though to our knowledge, there are no RCTs, and existing studies are largely ecological in design. The characteristics of arsenic-associated skin tumors include SCC in situ, SCCs, and BCCs (59–61). The first evidence of arsenic’s carcinogenic effects were among patients treated with arsenic-containing compounds for psoriasis, and then later in Germans exposed to arsenic containing pesticides (44,45). Eventually, several regions with high levels of arsenic contaminated drinking water revealed a dose-related relationship between arsenic exposure and KC (11). For example, in 1968, Tseng et al. conducted an ecologic analysis of arsenic-contaminated drinking water and KC prevalence among 40,421 residents in 37 villages of Taiwan’s Blackfoot disease endemic region and found an 8-fold difference in skin cancer prevalence between the highest level of arsenic exposure to the lowest, with an increasing trend in skin cancer prevalence from low to high (62). In a retrospective cohort study of Taiwan’s arsenic-endemic villages, skin cancer risk was related to the duration of living in the endemic area, duration of artesian well-water consumption, average concentration of arsenic in the drinking water, and an index for cumulative exposure to arsenic (Table 1b)(63).

The association between environmental arsenic exposure and KC has subsequently been reported in Asia, Eastern Europe, Latin America, and the US (36,42,56,64–67). Several ecological studies in endemic regions have found elevated standard mortality ratios (SMRs) of skin cancer among populations exposed to drinking water with high arsenic concentrations (11,37,68–73). In Chile, SMRs for KC have ranged from 3.2 (95% CI = 2.1 – 4.8) (73) to 7.7 (95% CI = 1.3 – 6.6) (37). In Taiwan, increased SMRs of skin cancer among people in arsenic-endemic areas have also been reported (68–72).

A summary of cohort, case-control, and cross-sectional studies of arsenic and skin cancer is in Table 1b. Multiple studies were conducted within Asian countries. Two hospital-based case-control studies in Taiwan revealed increased percentages of urinary methylarsonic acid (MMA), an organoarsenic compound commonly used in herbicides, and increased urinary levels of other arsenic species among patients with KC compared with controls (74,75). In the US, a population-based case-control study found no association between toenail arsenic levels and risk of SCC and BCC among residents in New Hampshire (76,77). In another population-based case-control study among residents in New Hampshire, a positive association was found between urinary measures of arsenic exposure and risk of SCC (77). A case-control study in Hungary, Romania, and Slovakia found a positive association between residential water arsenic concentration and BCC risk (78). Increased incidence and prevalence of KC has also been reported among residents of Wisconsin’s Fox River Valley, which contains arsenic-rich minerals in its bedrock layers (79,80), as well as in Eastern Europe (64), Mexico (81), and Vietnam (66). Taken together, epidemiologic studies from different geographical regions have consistently supported the positive association between arsenic exposure and KC risk. Studies which have evaluated BCC and SCC separately have reported similar positive associations.

Future directions for arsenic exposure and KC investigation include developing a better understanding of pathogenesis and genetic susceptibility. Individuals can vary in their susceptibility to arsenic toxicity (82). For example, only 15–20% of exposed individuals show evidence of arsenic induced skin damage (82). This variability may be influenced by a combination of inherited genetic factors and environmental and lifestyle factors. For example, the chromosomal region that contains the arsenite methyltransferase (AS3MT) gene is subject to multiple variants, which can affect an individual’s ability to metabolize and excrete arsenic (82–87). Variations in this gene could ultimately impact that individual’s susceptibly to arsenic exposure and degree of toxicity and carcinogenicity (82,86,87). As emerging data supports a role for genetic variation in arsenic metabolism, it may become a promising method of skin cancer risk evaluation.

Cadmium

Cadmium is a highly toxic heavy metal that is present in air, water, soil, sediment (11,88), and foods including green leafy vegetables (89–92). Cigarettes are a significant source of cadmium exposure (93). For non-smokers, diet and house dust are the main routes of cadmium exposure (94). Cadmium is considered a group I carcinogen by the IARC (12,93), and has been associated with tumors of the lung, testes, prostate, pancreas, adrenals, liver, kidney, blood, and pituitary (95).

As a carcinogen, cadmium’s mechanism of action is multifaceted, not fully understood, and ranges from aberrant gene expression (96) and errors in DNA methylation (97,98), to apoptosis blockage (99,100) and differentiation disruption (101). Cadmium can activate oncogenes and increase mitogenesis (102,103). Cadmium can also act in synergy with other human carcinogens like tobacco smoke and UVR (104). After exposure to UVR, cadmium can interfere with the removal of thymine dimers (104). Cadmium has been hypothesized to play a role in melanomagenesis through methylation and inactivation of Caspase 8 in the extrinsic apoptotic pathway (105). In uveal melanoma, cadmium has been found to alter the cell cycle through methylation and silencing of p16^INK4A (105,106). Absorption of cadmium has been found to be higher through the skin than in plasma (107), and is partly mediated through complexing with metallothionein, a heavy metal-binding protein involved in protective stress responses (108). Metallothionein overexpression in cancers has been implicated in poorer prognosis by anticancer drug and radiotherapy resistance (109,110).

Melanoma

The present epidemiological literature regarding cadmium exposure and risk of melanoma is limited. In an Austrian cohort study, metallothionein overexpression was a significant prognostic factor for primary melanoma patients (111). In an Italian case-control study examining trace elements in the toenails of 58 melanoma cases and 58 controls, higher levels of copper and lower levels of iron were found in patients with cutaneous melanoma, but no differences for cadmium (Table 2a) (25).

Table 2.

Epidemiologic studies of (^A) cadmium/chromium, (^B) iron, and (^C) copper exposure and cutaneous melanoma, and (^D) copper exposure and keratinocyte carcinoma listed by study design and year.

Reference	Study design	Demographics	Cases/controls or total participants	Exposures	Results (RR/OR/HR and 95% confidence interval)	Covariate Adjustment
A. Cadmium/Chromium
Melanoma
Cohort
Weinlich et al., 2003 (111)	Prospective Cohort	Sex: M/F Mean Age: 56.3 Country (Region): Austria (Innsbruck)	520 MM	Immunohi stochemic al overexpre ssion of metallothionein in melanoma	Progression: RR= 2.9 (1.46–5.76) Survival: RR = 4.19 (1.73–10.19)
Case-control
Vinceti et al., 2005 (25)	Case-control study (population-based)	Sex: M/F Age: NS Country (Region): Italy (Modena province) Ethnicity: NS	58MM/58 Controls matched by sex and age	Toenail cadmium concentration	For ≥ median cadmium levels compared to the remaining category: Cadmium : OR = 0.7 (0.3–1.9) Chromium : OR = 0.9 (0.2–3.2)	Education, sun exposure and total number of atypical nevi
B. Iron
Melanoma
Case-control
Stryker et al., 1990 (141)	Case-control study (hospital-based)	Sex: M/F Mean age: Cases M/F: 48/42 Controls M/F: 48/38 Country (Region): US (Massachusetts) Ethnicity: Caucasian	204MM/ 248 Controls visited dermatology clinic	Total iron intake	For highest quintile compared to lowest: OR = 0.8 (0.5–1.4)	Age, sex and total energy intake
Bain et al., 1993 (142)	Case-control study (population-based)	Sex: 41F Mean age: 50 Country (Region): Australia (Brisbane) Ethnicity: NS	41MM/ 297 Controls matched for age	Dietary iron intake	For highest tertile compared to lowest: OR = 0.39 (0.15–0.97) P = 0.04	Calories, age, number of painful sunburns, years of schooling
Vinceti et al., 2005 (25)	Case-control study (population-based)	Sex: M/F Age: NS Country (Region): Italy (Modena Province) Ethnicity: NS	58MM/58 Controls matched by sex and age	Toenail Iron concentration	For ≥ median levels of iron exposure compared to the remaining category: OR = 0.4 (0.1–1.4) Ptrend = 0.15	Education, sun exposure, and total number of atypical nevi
C. Copper
Melanoma
Case-control
Ros-Bullón et al., 1998 (157)	Case-control (hospital-based)	Sex: M/F Age: NS Country (Region): Spain (Murcia) Ethnicity: NS	35MM/39 Control serum obtained from healthy blood donors	Serum copper levels	Median copper levels: MM:118.3 ± 25.3 μg/dl Controls: 117.9 ± 28.0 μg/dl P > 0.05
Vinceti et al., 2005 (25)	Case-control study (population-based)	Sex: M/F Age: NS Country (Region): Italy (Modena province) Ethnicity: NS	58MM/58 Controls matched by sex and age	Toenail copper concentration	For ≥ median levels of copper exposure compared to the remaining category: OR = 15.5 (1.7–142.6)	Education, sun exposure and total number of atypical nevi
D. Copper
Keratinocyte Carcinoma
Case-control
Sahl et al., 1995 (158)	Case-Control (hospital-based)	Sex: M/F Mean age: 65 Country (Region): United States (South Dakota) Ethnicity: NS	46BCC/46 Controls matched by age, skin-type and sex	Mean daily copper intake	KC cases: 1.9 ± 0.1 mg Controls: 1.9 ± 0.9 mg P = 0.88
Vural et al., 1999 (159)	Case-Control (hospital-based)	Sex: 5M/7F Median age: 61 Country (Region): Turkey (Istanbul) Ethnicity: NS	12 BCC, 13 AK/16 Healthy controls matched by age, sex and average daily sun exposure	Plasma cerulopasmin	BCC: 219.1 ± 34.2(units/l) Controls: 251.7 ± 27.3 (units/l) P<0.05

Age given in years; RR = relative risk; HR = hazard ratio; OR = odds ratio; CI = confidence Interval; NS=Not specified

^{A, B, C.}MM = cutaneous malignant melanoma

^D.KC = Keratinocyte carcinoma; SCC = cutaneous squamous cell carcinoma; BCC = cutaneous basal cell carcinoma; AK = Actinic Keratosis

In experimental studies, metallothionein expression is associated with melanoma progression and has been suggested to be a poor prognostic indicator (109,112,113). In murine organ samples exposed to cadmium, melanoma cell invasion was enhanced through the induction of metallothioneins (110), suggesting a possible role for metallothioneins in malignancy and metastasis (110).

KC

Despite cadmium being considered a co-mutagen with UVR (104), relatively little has been studied with regard to cadmium and KC. To our knowledge, there are no epidemiologic studies investigating cadmium and KC. Lansdown and Sampson administered percutaneous cadmium chloride solutions to shaved rats and found dermal hyperkeratosis and acanthosis and increased mitotic indices in epidermal cells (114), suggesting a possible interaction between cadmium and keratinocytes (108,115).

Chromium

Chromium occurs primarily in the stable, nontoxic trivalent state (III), or in the strongly oxidizing hexavalent state (VI) (116). Humans are exposed to trace levels of chromium in the air, soil, water, and food including green beans, broccoli, high-bran breakfast cereals, and certain beers and wines (117). Hexavalent chromium is found mostly in air and water. While trivalent chromium is an essential trace metal, hexavalent chromium is a known carcinogen (13). The IARC concluded that chromium (VI) causes lung as well as nasopharyngeal cancers (11).

While it is unknown whether chromate can induce skin cancer, chromate does cause skin toxicity including allergic contact dermatitis and skin ulcers (118–122). Despite dermal exposure of workers to chromate, there are limited epidemiological studies evaluating chromate exposure and skin cancer (11,123). A population-based case-control study in Italy examined trace elements in the toenails of melanoma cases and controls and found no differences for chromium levels (Table 2a) (25). In a melanoma cell-line study, low concentrations of hexavalent chromium were found to increase cell proliferation (20). In a murine study, exposure to potassium chromate was associated with a dose-dependent increase in UV-induced SCCs (124,125). In the same study, chromium (IV) delivered in concentrations as low as 0.5ppm was able to induce skin tumors with UV, but chromate alone was a weak skin carcinogen (124,125). There is no human skin data about chromate and UV exposure (126). Further studies must be conducted to better understand the potential carcinogenic effects of chromium on skin.

Iron

Iron is the second most abundant metal on earth, after aluminum. Foods rich in heme iron include meats and fish, and nonheme sources including green leafy vegetables, legumes, and fortified foods (127). In humans, iron plays a key role in cell growth, respiration, and replication (128–131).

Iron is also involved in catalyzing redox reactions, which in the presence of UVA radiation, can produce reactive oxygen species (ROS) and play an important role in UVA-mediated skin cell damage (132). Iron could be carcinogenic due to its catalytic effect on the formation of ROS like hydroxyl radicals, suppression of host defense cell activity, and promotion of cancer cell multiplication (133–136). In both animals and humans, primary neoplasms have developed at sites of excessive iron deposits (136). Cancerous cells uptake iron at a higher rate (135,137,138), and generally have higher numbers of iron-binding cell receptors than their non-cancer counterparts (134,139,140). Despite potential links between iron and carcinogenesis, and iron and UVA-mediated skin damage, relatively little data exists about iron and skin cancer.

Melanoma

Only three epidemiological studies were found investigating iron exposure and risk of melanoma. A case-control study in the US investigated dietary intake of various vitamins and minerals, and found a non-significant inverse trend toward reduced risk of melanoma with increased dietary iron intake (Table 2b) (141). A case-control study in Australia evaluating nutrient intake also found an inverse association between dietary iron intake and risk of melanoma ( Table 2b) (142). Furthermore, an inverse association between toenail iron concentrations and melanoma risk was observed in an Italian case-control study (Table 2b) (25). These epidemiological studies contrast with experimental studies suggesting a possible protective role for iron in melanoma development (132). Further studies on the possible relation between reduced iron status and melanoma etiology are necessary.

KC

There are no epidemiologic studies investigating iron exposure and KC. In a study measuring levels of iron, copper, and zinc in the skin with noninvasive diagnostic x-ray spectrometry, all three elements were increased in both BCCs and SCCs compared with skin of healthy controls (143). In another histochemical examination of invasive BCCs and SCCs, only copper, not iron or zinc, were detected (144). In untransformed HaCaT and transformed A431 human keratinocytes, co-exposure with arsenic and iron was found to synergistically promote malignant transformation of untransformed keratinocytes, and progression of transformed keratinocytes (145). Despite possible associations between iron and UVA-induced skin damage, further studies are needed to elucidate a relation between iron and KC.

Copper

Copper is an essential trace element found in water and in certain foods including seafood, red meat, legumes, and whole grains (146,147). Copper plays a key role in many biological processes (148–154), often as an intermediate or cofactor in enzymes like cytochrome c oxidase and Cu/Zn superoxide dismutase (CuZnSOD) (148). Thus, copper contributes to mitochondrial ATP production and detoxification of reactive oxygen species (149–151). Copper also plays roles in gene expression regulation (148) and melanin formation (152).

Elevated serum and tissue copper levels have been observed in cancer patients, including breast, ovarian, hematologic, lung, colorectal, head and neck, and prostate suggesting altered systemic copper homeostasis (153). Copper promotes angiogenesis (154), activates enzymes involved in tumor cell migration and metastasis (154), and promotes oncogenic BRAF signaling and tumorigenesis (155). Given its contribution to cancer progression and increased uptake by malignant cells, cellular copper is a new potential target for novel anti-cancer therapeutics (154,156). Despite emerging research on cellular copper, relatively little is understood about environmental copper consumption and potential risk for skin cancers.

Melanoma

To our knowledge, there are only two small population-based studies on environmental copper exposure and risk of melanoma. An Italian case-control study found increased risk of melanoma with higher toenail copper levels (Table 2c) (25). Another case-control study in Spain found no association between serum copper levels and melanoma (Table 2c) (157). Further studies are needed to better elucidate a potential connection between copper consumption and melanoma.

KC

The epidemiological literature on copper and KC in humans is limited. In a case-control study, copper levels were examined in 46 patients with BCCs and controls, and no difference was found in dietary consumption of zinc or copper (Table 2d) (158). In another case-control study, ceruloplasmin, a major copper carrying protein in the blood, was noted to be decreased in patients with AKs and BCCs compared with controls (159). Further epidemiologic studies are needed to better elucidate the potential relationship between copper and KC.

Given the potential role of copper in tumorigenesis, as seen in other cancer patients, one would expect increased risk of skin cancer with increased copper levels (153). Studies measuring levels of copper in BCCs and SCCs both noninvasively and using histochemical techniques have detected increased amounts of copper compared with uninvolved skin or skin of healthy controls (143,144). A murine study noted incidental development of SCCs at or near sites of nickel-copper alloy ear tags in 8.8% of mice compared with 0% in the untagged ears, suggesting that chronic topical exposure to one or both of the metals is carcinogenic (21). Some studies have also suggested that a lower level of copper may confer a risk of KC, particularly lower levels of copper as a cofactor for antioxidant enzymes. Immunohistochemical stains of skin cancer biopsies have demonstrated reduced levels of CuZnSOD in AKs and SCCs, but increased levels in BCCs (160). Others have also found lower levels of CuZnSOD in SCCs and BCCs and surrounding tissues compared with younger-aged control skin (161). In a murine study, promotion and progression of papillomas, keratoacanthomas, and SCCs were found to be inhibited by pretreating with copper(II) (3,5-diisopropylsalicylate) 2, a superoxide dismutase agent with copper as a cofactor (162).

Zinc

Zinc is an essential trace element found in water, soil, foods including meat, eggs, whole grains, and dairy, building products, fertilizers, pesticides, and cosmetic products and sunscreen (163–166). Zinc is involved in over 200 enzymatic functions (167). At a cellular level, zinc is necessary for cell survival by playing key roles in signal transduction, transcription, and replication (168–170).

In cultured skin fibroblasts exposed to UVA and UVB, zinc protects against UV damage and reduces cytotoxicity and lipid peroxidation (171–173). When zinc was added to an immortalized human keratinocyte cell line, it decreased both the amount of DNA damage following UVB exposure and also the number of nucleosomes observed, a marker of apoptosis (174).

Topical zinc in the form of zinc oxide (ZnO) is an increasingly popular ingredient used in commercial sunscreen formulations for UV protection. Controversies regarding these nanoparticles involve concern of reactive oxygen species (ROS) development and penetration into the epidermis (175,176). There is conflicting evidence regarding absorption of zinc through the skin. Some in vivo and in vitro studies reported that nanoparticles are confined to the stratum corneum (175,177–179), while human studies have found increased amounts of zinc in blood and urine after ZnO sunscreen application (180,181). Longitudinal studies must be conducted on ZnO nanoparticles to better understand possible cytotoxic effects and long-term health implications. As of now, the health benefits of melanoma and KC risk reduction from sunscreen outweigh the current understood risk of these topical zinc formulations (182).

Melanoma

There are six epidemiologic studies on environmental trace zinc exposure and risk of melanoma. Some studies have found an inverse association between zinc exposure and risk of melanoma. In a US ecological study using state-averaged cancer mortality rate data for Caucasian Americans during 1970–94, indices for dietary zinc were found to be inversely correlated with melanoma mortality rate (183). In a population-based case-control study in the Czech Republic, lower serum zinc concentrations were found among subjects with melanoma (Table 3a) (184). In another case-control study, an inverse association was found between dietary zinc intake and risk of melanoma in Australians (Table 3a) (142).

Table 3.

Epidemiologic studies of zinc exposure and (^A) cutaneous melanoma and (^B) keratinocyte carcinoma

Reference	Study design	Demographics	Cases/controls or total participants	Exposures	Results (RR/OR/HR and 95% confidence interval)	Covariate Adjustment
Zinc
A. Melanoma
Case-control
Horcicko & Pantucek, 1983 (184)	Case-control study Population-based	Sex: NS Age: NS Country (Region): Czech Republic Ethnicity: NS	93MM/64	Mean serum zinc concentration	MM: 13.0 ± 2.4 μmol/l Controls: 17.0±2.8 μmol/l (P<0.01)
Gorodetsky et al., 1986 (143)	Case-control (hospital-based)	Sex: M/F Age: NS Country (Region): Israel Ethnicity: NS	71 samples (3 MM patients, 42 controls)	Wet weight concentration of zinc determined in vivo by diagnostic x-ray spectrometry	In malignant melanoma lesions: 13.9 ± 9.3ppm In uninvolved skin near melanoma lesion: 7.7 ± 3.5ppm In healthy controls: 6.7 ± 1.1 ppm (face and upper neck); 4.5 ± 1.7 ppm (chest, abdomen, arm, axilla, and lower neck)
Siu et al., 1991 (185)	Cast-control (hospital-based)	Sex: MM: 6M/16F Controls: 7M/10F Age: Adults, not otherwise specified Country (Region): NS Ethnicity: NS	22MM and 17 BCC as controls	Mean serum zinc levels	MM: 22.5 ± 1.2 μmol/l (mean levels) Controls: 17.6 ± 0.8 μmol/l (mean levels) P < 0.001
Bain et al., 1993 (142)	Case-control (population-based)	Sex: 41F Mean age: 50 Country (Region): Australia (Brisbane) Ethnicity: NS	41MM/ 297 Controls matched for sex and age	Dietary zinc intake	For highest tertile compared to lowest zinc intake and MM: OR = 0.36 (0.15–0.88) P=0.02	Calories, age, number of painful sunburns, years of schooling
Ros-Bullón et al., 1998 (157)	Case-control (hospital-based)	Sex: M/F Age: NS Country (Region): Murcia, Spain Ethnicity: NS	35 MM/39 controls Control serum obtained from healthy blood donors	Serum zinc levels	Median zinc levels: MM: 82.3 ± 25.34μg/dl Controls: 56.7 ± 11.8 μg/dl P < 0.0001
Vinceti et al., 2005 (25)	Case-control study (population-based)	Sex: M/F Age: NS Country (Region): Modena, Italy Ethnicity: NS	58/58 Controls matched for sex and age	Toenail zinc concentration	For ≥ median zinc levels compared to the remaining category and MM: OR = 3.5 (1.0–12.6) Ptrend = 0.48	Education, sun exposure and total number of atypical nevi
B. Keratinocyte Carcinoma
Case-control
Sahl et al., 1995 (158)	Case-control study (hospital-based)	Sex: M/F Mean age: Cases: 65 Controls: 64 Country (Region): US (South Dakota) Ethnicity: NS	46 BCC/ 46 Cancer-free controls matched by age, skin-type and sex	Mean daily zinc consumption	BCC: 12.2 ± 0.8mg Controls: 12.3 ± 0.7mg P = 0.71

Age given in years; RR = relative risk; HR = hazard ratio; OR = odds ratio; CI = confidence Interval

^A.MM = cutaneous malignant melanoma

^B.KC = Keratinocyte carcinoma; SCC = cutaneous squamous cell carcinoma; BCC = cutaneous basal cell carcinoma

Conversely, there are studies that have found positive associations between zinc exposure and risk of melanoma. Two hospital-based case-control studies found increased serum zinc concentrations among melanoma patients (157,185). Another hospital-based case-control study found increased zinc concentrations in melanoma lesions compared to uninvolved skin of cases and skin of healthy controls (Table 3a) (143). There are also studies that have found no significant associations between zinc intake and melanoma (Table 3a) (25).

KC

There is limited epidemiologic data regarding trace environmental zinc exposure and KC. In the same US ecological study investigating zinc and melanoma mortality, zinc and state-averaged KC mortality rate data was examined, and the dietary zinc index was also found to be inversely correlated with KC (183). In a case-control study, zinc levels were examined in patients with BCCs and cancer-free controls, and no difference in dietary consumption of zinc or copper was found between both groups (Table 3b) (158). Further studies are needed to better elucidate a potential connection between zinc exposure and KC.

Experimental study results are also mixed. As discussed with copper, immunohistochemical stains of AK, SCC, and BCC biopsies have shown reduced levels of CuZnSOD compared to skin of controls (161). Conversely, increased levels of zinc in BCCs and SCCs compared with skin of healthy controls has been demonstrated using noninvasive diagnostic x-ray spectrometry (143). In another histochemical examination of invasive BCCs and SCCs, zinc was not detected (144).

Selenium

Selenium is an essential trace element found mainly in soil, water, and foods including grains, mushrooms, asparagus, garlic, and animal products (186,187). Selenium has a narrow range for safe intake (188–190), and toxic levels (>400 μg/day) can induce alopecia, gastroenteritis (191,192), neurologic dysfunction (193–196), infertility, and dermatitis (197,198). The average content of selenium in the daily diet is far from the recommended amount (55 μg/day for persons 14 years or older in the US) (199), and 0.5–1 billion people worldwide are deficient in this metalloid (187,200). Selenium is genetically encoded into proteins as the amino acid selenocysteine; Selenium containing proteins include antioxidant enzymes that play essential roles in protecting against oxidation of lipid membranes, reduction of hydrogen peroxide, and organic peroxides (201–203). Selenium plays key roles in numerous essential cell and organ functions (202–208), and has been implicated in multiple diseases including diabetes mellitus (204,205) and cancer (206,207).

The association between selenium and cancer is controversial. Selenium has been implicated to have both anticancer and carcinogenic properties. Cancers that have been implicated involve nearly every organ system, including gynecologic, gastrointestinal, urinary, respiratory, hematological, endocrine, and skin (207,208). Limited skin cancer studies were included in these reviews. A recent meta-analysis on selenium exposure and cancer risk reported a pooled odds ratio of 1.09 (95% CI 0.98–1.21) for high selenium exposure and melanoma and KC combined, based on 6 effect estimates from 4 studies (206).

Melanoma

There have been 7 epidemiological studies of selenium and melanoma (Table 4a). In a US double-blind randomized placebo controlled trial among those with a history of cutaneous BCC or SCC, 200 μg/d of selenium supplementation was not effective in reducing melanoma risk (28). In two US prospective studies, no association was found between either toenail selenium concentrations or self-reported selenium supplement use and melanoma (209,210). Two case control studies similarly revealed no association between plasma and toenail selenium concentrations and melanoma (25,211,212). Conversely, some studies have found an association between selenium exposure and melanoma, though these studies in comparison to RCTs and cohort studies are more prone to bias given limitations in study design. An Italian prospective study found that exposure to tap water with high selenium levels was associated with melanoma risk (Table 4a) (213). In a case-control study, increased concentrations of plasma selenium were associated with increased risk of melanoma among an Italian population (214). In the same study, toenail and dietary selenium exhibited no evidence of a relation with melanoma risk; this difference could have been in part due to differences in specific selenium compounds (Table 4a) (214).

Based on these studies, selenium has not shown any beneficial role against melanoma risk. A few studies suggested potential adverse effects of selenium. In a murine study, a dose-dependent difference was found with selenium and melanoma development, with moderate dosage increasing tumor growth, and high dosage effectively treating and preventing recurrence of fully malignant tumors (215). In vitro studies have shown selenium inducing dose-dependent apoptosis in human A375 melanoma cell lines by inducing mitochondria-mediated oxidative stress (216). Taken together, these studies demonstrate the need to further investigate the exposure classification of selenium biomarkers, and metabolism of selenium to elucidate the potential relation between selenium exposure and melanoma risk.

KC

There are multiple epidemiologic studies investigating selenium exposure and risk of KC, including RCTs and prospective cohort studies. A double-blind RCT investigated whether 200 μg/d selenium as selenized yeast could prevent KC among BCC and SCC patients from the Eastern US (28). They found that selenium supplementation in fact elevated risk for SCC (relative risk [RR] = 1.25, 95% CI = 1.03 – 1.51) and total KC (RR = 1.17, 95% CI = 1.02 – 1.34), but not BCC (Table 4b) (28). A sub-study of the trial then tested 400 ug/d of selenium supplementation and found no effect, while those who continued to receive 200 μg/d of selenium maintained a higher risk of SCC (RR = 1.88, 95% CI = 1.28 – 2.79) and KC (RR = 1.50, 95% CI = 1.13–2.04) (Table 4b)(217). In a small trial among 184 French organ graft recipients, 200 μg/d selenium-supplementation had no effect on skin cancer (218). Case-control or cohort studies in the UK or US have not found an association between dietary, serum, or supplemental selenium and BCC (158,212,218–223) or SCC (212,220,223,224) (Table 4b). A meta-analysis evaluating selenium supplementation and cancer risk found non-significant positive associations between selenium and KC with 4 included studies (RR=1.23 [95% CI 0.73–2.08]) (207). In summary, the effect of selenium exposure on risk of KC is inconclusive despite relatively large numbers of existing epidemiological studies, while there is some suggestion of positive association with SCC risk.

The suggested positive association contradicts some experimental studies. Selenomethionine, a selenium organic compound, when applied topically for two weeks at increasing concentrations was effective in protecting against acute UV damage to the skin (225). In human keratinocytes, p53 activation was significantly diminished when incubated in selenomethionine both pre and post UVR irradiation (226). In another in vitro study with human keratinocytes exposed to UVR, a reduction in apoptosis was found by 71% when cells were incubated with selenomethionine or sodium selenite (227). A similar reduction in apoptosis had been noted in prior studies (228). Given selenium’s increasingly popular role as a dietary supplement (207) it is important to better understand the relation of this element to skin cancer.

Conclusion

Of all environmental trace elements, we identified published epidemiologic studies on exposure to arsenic, cadmium, chromium, copper, iron, selenium, and zinc and risk of skin cancer (Table 5). Some of these elements such as copper, iron, selenium, and zinc are essential and necessary for healthy biologic function. Other metals including arsenic, cadmium, and chromium are toxic and carcinogenic. Exposures to these metals are mainly through soil and water sources affecting foods and drinking water, as well as occupational, including pesticides, and field-specific activities such as welding and electroplating.

There were several epidemiological studies that reported a positive association between arsenic exposure and KC (both SCC and BCC), which was concluded as causally related with KC by the IARC. However, the studies on arsenic exposure and melanoma are still too limited to draw considerable conclusions.

Although biologically plausible, only a few epidemiological studies exist on exposure to cadmium, chromium, copper, iron, and zinc and skin cancer. Among them, cadmium and chromium are considered carcinogens for other cancers, but have insufficient evidence to conclude an association with skin cancer. While copper, iron, and zinc are essential nutrients in certain concentrations, they may adversely affect skin cancer at higher concentrations. Studies investigating exposure to zinc and risk of melanoma found associations in both directions. However, there is insufficient evidence to draw any definitive conclusions with no prospective data available on zinc and skin cancer.

Selenium has been more extensively investigated with both melanoma and KC. While selenium is hypothesized to reduce risk of other cancers, studies of selenium exposure and skin cancer risk did not find any inverse associations. A few studies, including evidence from RCTs, suggested a positive association between selenium exposure and KC risk.

In general, the literature on exposure to these elements and cutaneous malignancies has been quite limited, with studies of predominantly small sample sizes and study designs more prone to biases such as case-control and cross-sectional studies. It is necessary that more studies are conducted, with larger sample sizes and prospective study designs.

Effective methods to prevent and reduce environmental trace metal exposure requires sustainable broad public health initiatives, including testing drinking water sources and soil for heavy metal contamination, surveying vulnerable populations like pregnant women and at-risk workers (e.g. chromate plant workers) (229), creating and enacting legislation that bans pesticides with heavy metals and other toxins, and encouraging organic farming and dietary practices (230–232). For high-risk activities, enacting and enforcing strict clothing and equipment practices is necessary (233,234).

The current body of literature provides the groundwork from which future studies can build upon. In the setting of rising melanoma incidence and the markedly high prevalence of KC, it is imperative that environmental risk factors are identified and better understood for investigation of etiopathogenesis and preventative strategies.