Total mercury exposure through canned tuna in oil sold in Quito Ecuador

Abstract

Mercury (Hg) is a toxic element that bioaccumulates in aquatic organisms, posing health risks through seafood consumption. This study quantified total Hg in canned tuna in oil purchased from supermarkets in the Metropolitan District of Quito, Ecuador. Three commercial brands were analyzed according to market price (A < B < C) using a direct mercury analyzer. Mean Hg concentrations were 0.22 ± 0.10 mg/kg, 0.63 ± 0.10 mg/kg, and 0.36 ± 0.15 mg/kg for brands A, B, and C, respectively. All values complied with international safety limits; however, the non-carcinogenic risk (HQ > 1) associated with methylmercury exposure from brand C exceeded the reference threshold established by the U.S. Environmental Protection Agency. Recommended weekly intake of tuna to avoid health risks was estimated at 106 g (children) and 512 g (adults) for A; 26 g and 126 g for B; and 64 g and 307 g for C, respectively.

Introduction

Tuna is an important source of protein worldwide¹ and Ecuador is one of its major producers and exporters in Latin America^2–5. Accordingly, the tuna industry must follow norms for food quality and safety.

According to the Ecuadorian National Fisheries Institute, the main species of tuna found in Ecuador are yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus), and skipjack tuna (Katsuwonus pelamis)^3,4. Much of Ecuadorian tuna is canned in different oils (sunflower, olive, and vegetable) and water⁶.

Ecuador is on the red list of mercury (Hg)-emitting countries, releasing ~50 tons of this metal into the environment⁷. Natural and anthropogenic activities, particularly those from artisanal mining, as well as bioaccumulation, have caused these levels to increase⁸. Tuna is among the marine fish species able to bioaccumulate a substantial amount of Hg, in part because they filter an enormous amount of phytoplankton and are located at a higher level of the food chain. This is concerning, as it suggests that if Ecuadorian tuna is harvested from Hg-contaminated waters, consumers are being offered products of poor quality, which results in low food security.

Mercury is ubiquitous in the environment, and its bioaccumulation in fish depends on their size, diet, and trophic group⁹. Additionally, storage conditions for canned tuna, such as pH, oxygen concentration inside the can, and coating quality, can also increase Hg concentration¹⁰. Furthermore, the level of Hg toxicity in organisms may vary depending on Hg type and route of exposure. This metal is present in elemental, inorganic, and organic forms¹¹. In marine environments, inorganic Hg passes through organisms’ biological membranes and is transformed into methyl-mercury (Me-Hg), the most toxic form for the biota. The mercury methylation process begins with anaerobic bacteria; Me-Hg is subsequently assimilated by organisms from higher trophic levels through their diets, which causes biomagnification through food webs¹². Sulfate-reducing bacteria are key methylating agents.

The biomagnification of Me-Hg along the food chain results in elevated Hg concentrations in top predators^13,14. As species with a relatively long lifespan, tuna could also have high Me-Hg content. Approximately 80–90% of the Hg present in fish is Me-Hg, while the remaining 10% is inorganic Hg¹⁵. Absorption of Me-Hg via the gastrointestinal tract may be around nine times greater than that of its inorganic counterparts, which are absorbed in ~10% of the gastrointestinal tract. Therefore, consuming fish with high concentrations of Hg poses a considerable health risk^16,17.

The biological half-life of Me-Hg is ~65 days. Chronic Hg poisoning in an adult weighing 70 kg may result from the daily intake of ~0.3 mg of Me-Hg, which represents Hg concentrations of 0.2 mg/L in blood and 60 mg/kg in hair¹⁸. At the cellular level, the presence of Me-Hg can increase intracellular Ca²⁺, inhibit protein synthesis, disrupt microtubules, and lead to overproduction of free radicals because of the formation of reactive oxygen species in the kidney, liver, and brain^18–21. These chronic impacts on a living organism have long-lasting effects that can affect entire generations.

Various techniques have been used to quantify Hg in fish, including atomic absorption spectroscopy^22,23 neutron activation analysis²⁴, gas chromatography^25,26, inductively coupled plasma-mass spectrometry^23,27, high-performance liquid chromatography²⁷, atomic fluorescence spectrometry^23,28,29, and differential pulse anodic stripping voltammetry^30,31.

In 2006, the European Commission regulation established a maximum permissible concentration of Hg in food products of 0.5–1.0 mg/kg (fresh weight), which depends on the species, and a limit for food supplements that cannot exceed 0.1 mg/kg³². Recent large-scale compilations have shown that tuna Hg concentrations have remained relatively stable over several decades, underscoring the role of regional and historical biogeochemical processes in controlling fish Hg burdens³³. In parallel, new market-level surveys and risk assessments in Europe and other regions have reported elevated proportions of canned tuna samples approaching or exceeding regulatory benchmarks, renewing concern about consumer exposure and regulatory adequacy³⁴. Assessing Hg content in canned tuna has received little attention in Latin American countries, as it is widely assumed that its concentrations are low³⁵. In Ecuador, few studies have examined Hg content in canned tuna²². One study in 2015 quantified Hg in canned commercial tuna from the city of Manta in the Ecuadorian province of Manabí³⁶; cold vapor atomic absorption spectroscopy showed Hg did not exceed Ecuador’s permissible limits and Aguilar-Miranda et al.²⁹, reported using atomic fluorescence spectroscopy that the Hg content of canned tuna in water also did not exceed standards, however, the amounts of Hg in those samples were found to be risky to health.

Extending these previous investigations, the present study quantified total mercury (THg) in canned tuna in oil from the most widely consumed brands in the Ecuadorian market. In addition, the measured THg concentrations were compared with current national and international regulations on permissible Hg limits in food products and with values reported in the literature to assess compliance and contextualize the findings. Specifically, the study aimed to: quantify THg using a direct mercury analyzer (DMA-80) following EPA Method 7473; evaluate compliance with Ecuadorian and international regulatory thresholds; compare the results with regional and global reports on Hg in canned tuna; estimate consumer exposure through the maximum weekly intake and hazard quotient (HQ) for adults and children; and provide evidence to support public health authorities in strengthening risk assessment and regulatory oversight in Ecuador.

Results and discussion

Quantification of total mercury content

Considering the maximum permissible Hg levels established by the European Union (EU), the United States (US), and the Canadian Food Inspection Agency (CFIA), the mean THg concentrations reported in Table 1 for the analyzed tuna brands were all below the corresponding regulatory thresholds. Food safety authorities have implemented various regulations and guidelines to control Hg concentrations in canned tuna to safeguard public health. Although these regulations differ among countries and agencies, their common objective is to minimize dietary Hg exposure from tuna consumption. Under Regulation (EC) No. 629/2008, the EU established maximum Hg limits of 1.0 mg/kg (w.w.) for predatory species and 0.5 mg/kg (w.w.) for other fish species. The U.S. Food and Drug Administration (FDA) set a limit of 1.0 mg/kg for tuna, whereas the U.S. Environmental Protection Agency (EPA) recommends 0.5 mg/kg (w.w.) as the safe level for human consumption. Similarly, the CFIA specifies a limit of 0.5 mg/kg (w.w.) for canned tuna and 1.0 mg/kg (w.w.) for fresh or frozen tuna.

Table 1.

Total mercury in the three Brands of canned tuna analyzed

Total mercury levels (mg/kg)*
Brand A	Brand B	Brand C
0.22 ± 0.10 (0.05–0.40)	0.63 ± 0.10 (0.48–0.89)	0.36 ± 0.15 (0.14–0.61)

^* Values are presented as mean ± standard deviation (range).

According to the boxplot analysis (Fig. 1), the data distribution for Brand B was negatively skewed, as the mean was greater than the median and the mode; while for Brand C, the distribution was positively skewed. For brand A, the distribution was symmetric because the mean and median are very close (Gaussian behavior). For brand B, 75% of the data indicated that the THg content was between 0.48 and 0.67 mg/kg; for brand A, the content was between 0.05 and 0.23 mg/kg and for brand C, between 0.14 and 0.50 mg/kg. Brand B, which corresponds to the intermediate price in the Ecuadorian market, had an average THg value of 0.63 mg/kg and was therefore the closest to exceeding the Ecuadorian limit of 0.50 mg/kg; Brand A, which sold at the lowest price on the market, had the lowest THg content. For the Brand with the highest market price, an intermediate THg content level was found compared to the other two brands.

Fig. 1.

Box plot of the total mercury content (mg/Kg), for the three analyzed Brands of canned tuna in oil.

Kruskal–Wallis test revealed statistically significant differences in THg concentrations among the three canned tuna brands analyzed (p < 0.01). Subsequent pairwise comparisons using the Wilcoxon test indicated that brand B’s THg levels were significantly different from those of brands A and C. This distinction appears to be influenced by two specific batches from brand B that exhibited the highest THg concentrations. The p-values for brands A, B, and C were 0.1228, 0.3788, and 0.4035, respectively, according to the Kruskal–Wallis test.

Chebyshev’s theorem was applied to estimate the confidence intervals and assess the degree of dispersion within each dataset. This theorem allows for calculating the minimum proportion of population values that lie within k standard deviations from the mean using the expression $(1-\frac{1}{k^2})$, and conversely, the maximum proportion of values falling outside this range using ($\frac{1}{k^2}$), where k represents the number of standard deviations and must be greater than 1. In Table 2, the confidence intervals were computed using specific k values: π, φ, and e, which were chosen for their relevance in modeling natural and artificial systems^37,38. When k was set to π (3.1416), Chebyshev’s theorem predicted that at least 89.87% of the data fell within that range from the mean. For e (2.72), the corresponding coverage was 86.47%, and for φ (1.618), it dropped to 61.80%. Moreover, the results shown in Table 2 indicate that brand C exhibits the widest confidence interval among the analyzed brands. According to Chebyshev’s theorem, if new observations are added to the population of brand C, there is an 89.87% likelihood (with k = π) that these new data points will fall within π standard deviations of the mean. This supports the conclusion that brand C consistently presents intermediate THg levels compared to brands A and B, with a higher degree of statistical confidence.

Table 2.

Confidence intervals, according to uncertainties about the distribution of the data, obtained from Chebyshev’s theorem

Standard deviations	Minimum % within [1-(1/k2)]	Max % outside [1/k2]	Brand A confidence interval	Brand B confidence interval	Brand C confidence interval
π (3.141593)	0.8987	0.1013	(−0.16769, 0.2870796)	(−0.227532, 0.674699)	(−0.05291418, 0.4118852)
e (2.71828)	0.8647	0.1353	(−0.152221, 0.302559)	(0.2427424, 0.6899189)	(−0.03710402, 0.4277052)
φ (1.618034)	0.6180	0.3197	(−0.040059, 0.386385)	(0.3530297, 0.7723444)	(0.07753211, 0.5133809)

Table 3 compares the THg concentrations obtained in this study with values reported in the published literature. Overall, the concentrations observed here fall within the ranges documented by most previous investigations, with the exception of the unusually elevated levels reported for samples from México, which suggest a potential exceedance of the permissible Hg limits for canned tuna in that region. Our findings are broadly consistent with recent national and regional surveys, which generally report mean THg concentrations below international regulatory thresholds but acknowledge the occurrence of sporadically high values; similar patterns have been documented for canned tuna sold in Quito^29,31. Nevertheless, large-scale compilations demonstrate that tuna Hg levels exhibit marked geographic variability and temporal stability, underscoring the importance of continued local monitoring and the implementation of targeted regulatory measures³³.

Table 3.

Studies on THg content in canned tuna in water and oil performed in other countries

Type of preserved canned	City / Country of study	Origin of samples	Species	Technique	Mean Total Hg (mg/kg)	References
Conserved in water	New Jersey, Unites States	United States	Thunnus germo and Katsuwonus pelamis	Cold Vapor Atomic Absorption Spectrophotometry (CV-AAS)	0.10 to 0.4	57
	Lower Austria, Burgenland and from Viennese retail operations	Different brand	Tuna not specified		0.14	58
Not specified	Las Vegas, United States	Not specified	Tuna not specified	Atomic Absorption Mercury Analyser (AMA)	0.54 to 0.71	59
Not specified	Tyrrhenian Sea	Not specified	Tuna not specified	Atomic Absorption Spectroscopy (AAS)	0.04 to 1.79	60
Conserved in water and oil	Mazatlán, México	México	Thunnus albacares	Cold Vapor Atomic Absorption Spectrophotometry (CV-AAS)	0.25 to 0.51	10
Olive oil, sunflower seed oil, water or marinade	Spain	The most popular brands in Spain	Yellowfin, bigeye tuna, and skipjack tuna	Atomic Absorption Spectrometry andThermal Decomposition Amalgamation		61
Not specified	Italy, Libya, Spain and Thailand	Different brand origin and country of manufacture	Tuna skipjack and yellowfin	Direct Mercury Analyzer (DMA)	0.16	62
	Spain	Spain	Thunnus albacares and Thunnus alalunga	Inductive Coupled Plasma-Mass Spectrometry (ICP-MS)	0.33 ± 0.15	63
Conserved in oil	Sao Paulo, Brazil	Not specified	Tuna not specified	Thermal Decomposition, Amalgamation and Atomic Absorption Spectrometry (TDA- AAS)	0.04 to 0.40	64
Conserved in water	Sao Paulo, Brazil	Not specified	Tuna not specified	Thermal Decomposition, Amalgamation and Atomic Absorption Spectrometry (TDA-AAS)	0.05 to 0.46	64
Conserved in water	Vojvodina, Serbia	Thailand, Vietnam, Indonesia and Spain	Tuna not specified	Inductive Coupled Plasma-Mass Spectrometry (ICP-MS)	0.010 to 0.64	65
Conserved in water	Portugal	Caught by commercial vessels from the Eastern Central Atlantic Ocean	Skipjack tuna	Thermal Decomposition, Amalgamation and Atomic Absorption Spectrometry (TDA-AAS	0.13	66
Conserved in water	Cartagena, Colombia	Ecuador and Colombia	Tuna not specified	Direct Mercury Analyzer (DMA)	<0.001 to 0.86	50
Conserved in oil	Cartagena, Colombia	Supermarkets located in Cartagena (10°25′25″N 75°31′31″W) on Colombia’s northern coast	Tuna not specified	Direct Mercury Analyzer (DMA)	∼0.61	50
Not specified	Iran	Purchased from 10 different markets in Tehran	Thunnus albacares, (Clupeonella cultriventris caspia, Euthynnus affinis, Thunnus tonggol	Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)	0.13	67
Conserved in water	Ecuador	Not specified	Tuna not specified	Cold Vapour Atomic Absorption Spectrophotometry (CV-AAS)	0.02 to 0.09	68
Conserved in oil	Serbian	Not specified	Tuna not specified	Coupled Plasma-Mass Spectrometry (ICP-MS)	0.03 to 0.07	69
Shredded in vegetable oil	Barranquilla, Colombia	China, Ecuador, and Colombia	Thunnus albacares	Cold Vapor Atomic Absorption Spectrophotometry (CV-AAS)	0.13 to 1.47	35
Conserved in oil	Iran	Persian Gulf	Tuna not specified	Atomic Absorption Spectrometer (FAAS)	177–315,3 ppb	70
Not specified	Tijuana, Mexico	Not specified	Thunnus albacares	Cold Vapor Atomic Absorption Spectrophotometry (CV-AAS)	0.01 to 1.17	38
Not specified	Italy	Different brands sold in different markets	Tuna not specified	Standard methods: EN 13805:2014	0.21	71
Conserved in oil	Italy	Italian market	Tuna not specified	Atomic Absorption Spectroscopy (AAS)	0.04
Conserved in water and oil	Ecuador	Eastern Pacific international waters	Katsuwonus pelamis, Thunnus albacares, and Thunnus tuna	Cold Vapor Atomic Absorption Spectroscopy (CV-AAS)	∼0.04	6
Not specified	Foggia	Italy	Tuna not specified	Inductive Coupled Plasma-Mass Spectrometry (ICP-MS)	0.21 ± 0.18	71
Conserved in water	Quito	Ecuador	Tuna not specified	Cold Vapor Atomic Fluorescence Spectrophotometry (CV-AFS)	0.02 to 1.98	29
Conserved in water	Canary Islands	Spain	Tuna not specified	Cold Vapor Atomic Absorption Spectrophotometer	0.01 to 0.86	34
Conserved in oil	Quito	Ecuador	Tuna not specified	Direct Mercury Analyzer	0.22 to 0.63	This study

Regarding the medium in which the tuna is preserved inside the cans, the results indicate no a priori difference in THg content between samples of the same brands preserved in water, according to the report by Aguilar-Miranda et al. ²⁹, and those preserved in oil, according to the present study. The possible differences between these results and those from the studies presented in Table 3 may be due to differences in species, size, fish lifespan, and capture site. For example, according to Farrell Anthony³⁹, skipjack tuna (K. pelamis), which is highly popular in terms of fishing and represents 58% of global tuna catch⁴⁰, has a shorter lifespan and therefore accumulates less Hg in its tissues. Thunnus alalunga reaches a maximum weight of ~40 kg at 15 years old, while K. pelamis has a maximum weight of ~30 kg at the same age.

In canned tuna, between-brand differences in THg are best explained by sourcing and processing factors rather than fish age or size, which are typically unknown and unlikely to systematically vary at the can level. First, brand-specific sourcing (species composition and geographic catch origin) is a primary driver of Hg variability; within a given species, geographic origin has been shown to markedly influence Hg burdens, and brand effects have been reported even after accounting for product type⁴¹. Second, processing-related changes in moisture/lipid content during canning can increase measured Hg concentrations on a wet-weight basis by concentration (dehydration) effects, while the packing medium (oil vs. water) and draining generally have limited or inconsistent influence on Hg levels in the edible portion⁴². Finally, because Me-Hg constitutes the dominant fraction of THg in tuna muscle, THg remains an appropriate proxy for exposure assessment in canned products⁴³.

Considering the widespread consumption of canned tuna globally and the potential Hg-related side effects for human health, the species, size, and catch location should be required and regulated on labels. In this regard, Shim et al.⁴⁴ suggested that, because of young children’s susceptibility to Hg’s toxic effects, products with low metal concentrations should be specially labeled as “safe for children”; they proposed that the maximum Hg content for commercial fish be reduced to 0.185 mg/kg. Generally, this information is confusing or insufficient on such labels and limits a more consistent assessment when comparing results between different regions. Since December 13, 2014, the EU has implemented changes in the regulations for labeling all fishery and aquaculture products. For products such as canned fish, information on the capture zone shown in the barcodes is voluntary, unlike for unprocessed fish products, for which the capture zone must be specified; however, many retailers have chosen to provide this information⁴⁵. Given the widespread consumption of canned tuna and the potential health risks, it is recommended that product labels include information on species, size, and catch location. In this context, Shim et al.⁴⁴ proposed that products with low Hg levels should be specially labeled as “safe for children”. In Ecuador, labeling for these products currently only includes nutritional information.

Human health risk assessment

In marine environments, inorganic Hg passes through biological membranes and transforms into Me-Hg, which could be an indication of a biomagnification process of Hg in the food web, given that Me-Hg is highly lipophilic. The study methodology used allowed THg values to be measured only in the samples. However, the literature has suggested that, depending on the species, particularly for fish such as tuna, when analyzing muscle tissue, Me-Hg constitutes a significant percentage of THg content, ranging from 80 to 100% in some cases⁴⁶.

Table 4 shows the results obtained of the exposure levels, potential non-carcinogenic risk, and recommended weekly intake of fish meat, for children and adults, based on mean content of THg found in the three brands of canned tuna in oil. The level of exposure to THg (Ex) calculated, in none of the brands exceed the FDA reference dose of 0.1 mg/kg day BW^-147,48. However, for an adult weighing 70 kg and a child weighing 14.5 kg, the non-carcinogenic potential risk (Rx) values were greater than or close to 1 for Brand C, 1.4 and 0.6, respectively, suggesting that consuming tuna from this brand poses a health risk.

Table 4.

Exposure levels (Ex), potential non-carcinogenic risk (Rx), and maximum weekly intake of fish meat for children and adults, based on mean content of MeHg found in the three brands of canned tuna in water

	Brand A				Brand B				Brand C
	Ex (mg kg−1 day BW−1)	Rx	Maximumweekly intake (g fish/week)	HQ*	CRmw	Ex (mg·kg−1 day BW−1)	Rx	Recommended weekly intake (g fish/week)	HQ*	CRmw	Ex (mg·kg−1 day BW−1)	Rx	Recommended weekly intake (g fish/week)	HQ*	CRmw
Childrena	8.62×10−5	0.9	106	9.65	19.32	2.48×10−4	2.5	26	27.78	6.71	1.44×10−4	1.44	64	16.10	11.57
Adultb	3.57×10−5	0.4	512	1.99	4.00	1.03×10−4	1.0	126	5.75	1.39	5.90×10−5	0.60	307	3.34	2.40

Provisional tolerable weekly intake (PTWI) (µg MeHg·kg^-1 HBW): 1.6.

^a Body weight of 14.5 kg for children; ^b Body weight of 70 kg for adults.

*Hazard quotient (HQ).

Based on these results, the recommended weekly intake of fish meat for children and adults was calculated. According to the FDA/EPA, 2022⁴⁹, the highest allowable THg concentration in fish when eating 1 serving per week is 0.46 mg/kg. The suggested weekly consumption values, calculated from the highest THg concentration found for each brand for children (body weight 14.5 kg) and adults (body weight 70 kg), were 106 and 512 g fish/week for brand A; 26 and 126 g fish/week for brand B; and 64 and 307 g fish/week for brand C, respectively. These results suggest that consuming canned tuna in oil in higher quantities could have negative consequences for human health³⁴. Brand A allows for the highest weekly consumption. Consuming more than six 80 g can of tuna with a higher Hg content per week may pose a risk to Ecuadorian consumers.

According to Ormaza et al.⁶, canned tuna in Ecuador is consumed once or twice a week, although not consistently. Assuming a weekly portion of 80 g for adults and 40 g for children (half a can), the non-carcinogenic risk index (Rx) calculated for the average THg concentration in brand C exceeds 1. This suggests a significant potential risk when consuming more than one portion per week, particularly for vulnerable populations such as pregnant women and children. For brands A and B, the risk is lower, though children remain more vulnerable than adults do. In other countries, such as México, studies have reported low non-carcinogenic risks associated with canned tuna consumption; however, consumption levels vary greatly. For example, average daily tuna intake is 3.92 g in México and 5.71 g in Italy, while in Ecuador; the estimated daily fish consumption is 37.0 g, though it’s not specified whether this includes canned tuna. Therefore, frequent consumption of tuna or other fish species should be a health concern. Notably, the FDA (2008) also reported considerably lower Hg limits for canned albacore and skipjack tuna, 0.035 and 0.0112 mg/kg, respectively. Based on these specific thresholds, all brands analyzed in this study contained Hg concentrations substantially higher than those values. Nevertheless, according to the Ecuadorian National Fisheries Institute, these particular species are not used in Ecuadorian canned tuna products, although this discrepancy remains a matter of concern for consumer protection and regulatory harmonization.

The estimated risk of THg exposure, using the hazard quotient (HQ) value and mean concentration of Me-Hg (0.80xTHg) in the samples⁵⁰, was much higher than 1 for all brands (Table 4), indicating possible adverse health effects for consumers. Similarly, using the calculated mean Me-Hg concentration in each sample, the Table 4 shows the recommended maximum allowable rate of consumption of tuna in meals/week, CRmw⁵¹. This is also graphically presented in Fig. 2 as HQ vs. CRmw; the results confirm that indeed the brand A could be consumed in a greater number of meals per week.

Fig. 2.

Relationship between Hazard Quotient (HQ) and maximum weekly consumption ratio (CRmw) per Brand.

Among the three canned tuna, brands analyzed, also among the most consumed in Ecuador, brand B exhibited the highest THg concentration, reaching up to 0.63 mg/kg. Despite this, the majority of samples from brands A, B, and C presented THg levels below the maximum limits established by national and international food safety standards.

Risk assessment based on the estimated Me-Hg intake (Rx) indicated that consuming a single weekly serving of 80 g for adults and 40 g for children does not represent a significant non-carcinogenic health risk. However, increasing portion size or consumption frequency may pose a greater risk for vulnerable groups, especially children, due to their lower body weight and developing systems. While fish intake is nutritionally beneficial, its consumption must be balanced to minimize potential mercury exposure.

Given the potential health implications, particularly for pregnant women and children, it is strongly recommended to implement national strategies for routine monitoring of mercury levels in canned tuna and other commonly consumed fish products. Additionally, Ecuador’s regulatory framework should mandate the labeling of tuna species and catch origin on packaging, as these factors are critical for assessing risk. Improved labeling would empower consumers to make informed choices and avoid species known for higher mercury accumulation.

This study provides relevant evidence on THg levels in the most widely consumed canned tuna brands in the Ecuadorian market, highlighting the need to strengthen food safety monitoring and surveillance systems. Although risk assessment for Me-Hg exposure suggests that occasional consumption within recommended portion sizes does not pose a significant threat to the adult population, cumulative exposure in vulnerable groups such as children and pregnant women may present a public health concern. Therefore, it is imperative to promote policies that support sustainable fishing practices, improved labeling standards, and public education campaigns aimed at encouraging safe and informed fish consumption across the country.

Methods

Sampling

From March 2023 to March 2024, sixty cans of 80-gr of tuna were collected from local supermarkets on the south and north sides of Quito - Ecuador. Brands were coded as A, B, and C according to the price at which they are sold in the market (A < B < C). Twenty cans of solid tuna in oil were collected for each brand; each can belonged to different batches. The canned tuna samples must follow national (e.g., the Ecuadorian Standards Institute, Ministry of Agriculture and Livestock, National Fisheries Institute) and international (e.g., WTO, CIAT, FDA, WHO, FAO, ISO, IFS, BRC, dolphin-safe, halal, kosher) quality and safety standards and labels and must have a valid sanitary registration.

Sample treatment

Upon opening each can, the packing medium was carefully removed, and the edible portion of the tuna was collected. The solid fraction was subsequently homogenized using a stainless-steel cutting system (Oster Classic 4655, Oster, USA) to obtain a uniform matrix suitable for analytical determination. Homogenization was performed to minimize within-sample variability and ensure representative subsampling for mercury analysis. From each can, 60 g (Analytical balance Shimadzu AUW220D, Shimadzu Corporation, Japan) was taken and weighed in petri dishes, placed in an ultra-freezer for 1 h, and lyophilized (Freeze dryer, Labogene, Bjarkesvej 5, Denmark) for 48 h until a constant weight was achieved. The percentage moisture content of each can was calculated.

Quantification of total mercury

Total mercury (THg) concentrations were quantified using a direct mercury analyzer (DMA-80, Milestone, Italy), which operates through thermal decomposition of the sample, subsequent mercury amalgamation, and atomic absorption detection. This approach enables the direct measurement of mercury in solid food matrices without the need for chemical digestion, following the principles established in EPA Method 7473⁵². Approximately 40 mg ± 5 mg of dried sample were placed into a nickel sample boat and introduced into a catalytic furnace.

Analytical reliability was verified through a comprehensive quality assurance protocol that included procedural blanks, replicate analyzes (n = 15), and the periodic measurement of certified reference materials (DORM-4, fish protein) after every 10 samples. Method performance was assessed by evaluating precision and accuracy, confirming the suitability of the analytical procedure for THg determination in canned tuna matrices. These results were considered reliable, as the precision and accuracy values were within the limits permitted by the AOAC (“AOAC SMPR 2012.007: Standard Method Performance Requirements for Determination of Heavy Metals in a Variety of Foods and Beverages,” 2013).

Risk assessment

Consumer health risk was assessed for different THg concentrations (minimum, mean, and maximum). All calculations were performed using Microsoft Office Professional Plus Excel 2016.

The level of exposure (Ex) to Me-Hg was calculated using the following equation from the US EPA:

$$\mathit{Ex}=\frac{\mathrm{Cx}\times \mathrm{CR}}{\mathrm{BW}}$$ 1

where Cx is the concentration of metal in the edible portion of the samples (mg/kg), CR is the mean amount of tuna (kg) consumed daily, and BW is the consumer’s body weight (kg)⁵³ (US-EPA, 2000). The potential non-cancer risk (Rx) from consumption was determined using the following Eq. 2⁵⁴:

$$\mathit{Rx}=\frac{\mathit{Ex}}{\mathrm{RfD}}$$ 2

where Ex is the exposure to the pollutant (mg / (kg‧d)), and RfD is the reference dose of the chemical (mg/(kgd))⁵⁴.

Considering the Provisional Tolerable Weekly Intake (PTWI) for methylmercury (MeHg) of 1.6 μg/kg of human body weight (HBW), as established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA)⁵⁵, the maximum weekly intake of canned tuna (in grams per week) was estimated using the the following Eq. 3:

$$\mathrm{Intake}\left(\mathrm{g}\right)=\mathrm{PTWI}\times \mathrm{HBW}/\mathrm{Cx}$$ 3

Where Cx is the average concentration of MeHg (mg/kg) determined for each canned tuna brand in this study. For the purposes of this calculation, the HBW was set at 70 kg for adults and 14.5 kg for children.

Data analysis

Mercury concentrations are reported relative to the original sample mass and expressed on a wet weight basis. Descriptive statistics, including the mean, standard deviation, range, and recovery, were calculated using Excel 2016. Data distribution was examined through boxplot visualization and by applying Chebyshev’s theorem⁵⁶, which was implemented using R software. Differences in THg concentrations among the three canned tuna brands were evaluated using non-parametric statistical analyzes, specifically the Kruskal–Wallis test followed by pairwise Wilcoxon post hoc comparisons.

Author contributions

E.T.C.: data curation and methodology; L.F.: contributed to funding acquisition, conceptualisation, data curation, formal analysis, methodology, project administration, supervision and writing; N.C.S.: contributed to data acquisition, data curation, methodology and supervision; M.R.U.: contributed to data acquisition, data curation and methodology; E.O.M.M.: contributed to formal analysis; D.B.M.: contributed to data curation and methodology; P.E.M.: contributed to data curation and writing.

Data availability

Data “available on request”: lmfernandez@puce.edu.ec.

Competing interests

The authors declare no competing interests.