Money and Microbes: A Global Systematic Review and Meta-Analysis of Currency Contamination

Abstract

Money passes through numerous hands daily, creating an ideal medium for the accumulation and transfer of pathogenic microorganisms. Despite this, a comprehensive synthesis of contamination levels on currency is lacking.

Aim:

This systematic review and meta-analysis assessed the prevalence and types of microbial contamination on money.

Methods:

A systematic search was conducted for studies published up to December 2024. The review followed the PRISMA guidelines. Pooled prevalence estimates were calculated via a random-effects model. Heterogeneity between studies was assessed via the I² statistic, and sensitivity analyses were performed.

Results:

A total of 35 studies met the eligibility criteria. The overall pooled prevalence of microbial contamination on money was 12.8% (95% CI: 10.9-14.7). Subgroup analyses revealed pooled prevalence rates of 14.0% (95% CI: 11.1-16.8) for bacteria, 17.0% (95% CI: 12.1-21.8) for fungi, and 9.7% (95% CI: 6.8-12.7) for parasites. Klebsiella spp. 20.3% (95,% CI: 10.4-30.2) and Staphylococcus aureus 19.8% (95% CI: 12.7-26.8) were the predominant bacterial isolates, whereas Aspergillus niger 42.2% (95% CI: 29.1-55.3) was the most frequently identified fungal species, and Entamoeba histolytica was the most frequently identified parasites 18.9% (95% CI: 1.3-36.5). Contamination was greater for paper money 21.3% (95% CI: 13-29.6) than for coins 14.8% (95% CI: 4.3-25.3). Specifically, bacteria were more common on coins 26.4% (95% CI: 8.6-44.2) than on paper money 23.7% (95% CI: 9.5-37.8), whereas parasites were more common on paper money 18.1% (95% CI: 1.2-15.4) than on coins 2.1% (95%, CI: 1.2-3.0). The microbial load of the dirty notes was more 3.0% (95% CI: 2.3-4.1) and that of the newer notes was 2.2% (95% CI: 1.3-3.7).

Conclusion:

Money is consistently contaminated with potentially pathogenic microbes. Although direct transmission has not been definitively proven, the frequent presence of clinically significant microorganisms on money poses a credible public health risk.

Plain language summary

How dirty is our money? A global review of microbes found on currency

Money is one of the most frequently exchanged items in daily life, yet it is rarely cleaned or disinfected. This global review analyzed studies from multiple countries to determine how often money is contaminated with microbes that can cause disease. The findings revealed that more than 1 in 10 currency samples tested (12.8%) carried bacteria, fungi, or parasites. Fungi were the most common group found, followed by bacteria and parasites. Some of the identified microorganisms can be harmful, particularly for people with weakened immune systems, although this study did not assess whether handling money directly causes infection. Contamination was greater for paper money than for coins and for older, worn-out notes than for newer notes. These findings highlight the importance of washing hands after handling cash and supporting safer practices such as digital payments and cleaner banknote materials.

Introduction

Money, whether in the form of coins or paper currency, is one of the most widely handled items globally, facilitating countless daily transactions across socioeconomic and cultural settings.^1,2 Owing to its ubiquity and frequent handling, money serves as a potential reservoir and vehicle for microbial contamination, harboring various pathogens, including bacteria, viruses, fungi, and protozoa.^{3 -7} The association of money with disease transmission is not a new concept. Historically, currency has been implicated in the spread of deadly pandemics, such as the “black death” or bubonic plague, which devastated populations in medieval times.^3,8 However, despite these longstanding concerns, the global extent, distribution, and organism-specific patterns of microbial contamination on currency remain poorly synthesized, with evidence scattered across regions and pathogen groups.

In modern times, concerns about microbial contamination have intensified, particularly during the COVID-19 pandemic, which highlights the significance of the surface-to-human transmission of pathogens. ⁹ First, paper bills offer a large surface area for organisms and organic debris to collect. ¹⁰ Second, folds and/or deliberate depressions or projections specifically engineered into bill designs as anticounterfeiting methods serve as settling sites for both organisms and debris, which allows microorganisms to live longer. ¹¹ Finally, banknotes weave their way through the population for many years before they come to rest. ¹¹

The extent and type of microbial contamination of money are influenced by several factors, including environmental conditions, seasonal variations, material composition, and hygiene practices within the population. Studies indicate that the age and denomination of a bill are directly correlated with the degree of contamination observed. ¹² Dirty or damaged notes, for example, harbor significantly higher microbial loads than clean or mint notes do.^{13 -15} Local community flora and socioeconomic activities also contribute to the microbial diversity present in currency, further complicating its role in disease transmission.^16,17

The possibility that currency might serve as a vehicle for transmitting pathogenic microorganisms was first suggested in the 1970s. ¹⁸ Since then, a growing body of literature has documented the presence of diverse microorganisms in currency notes.^3,5,6,16 These pathogens contribute significantly to the global antimicrobial resistance crisis, which has claimed millions of lives annually despite significant investments in healthcare interventions.^19,20 However, despite decades of research, a global synthesis of these findings is lacking. Data from such studies are essential in the development of public health policies that will help reduce the spread of pathogens through money, particularly in regions where cash continues to play a central role in daily transactions. To fill this gap, we conducted the first global systematic review and meta-analysis aimed at comprehensively evaluating the prevalence and types of microbial contamination on money and possible mitigation strategies to reduce the public health burden.

Methodology

This systematic review was conducted and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. ²¹

Search Strategy

A systematic and exhaustive search of the relevant literature was conducted between November 15th and 26th, 2024. The electronic databases PubMed, Web of Science, Scopus, and Google Scholar were systematically explored, with a focus on studies published up to December 2024. The search on Google Scholar was limited to the first 6 fields. Keywords and Medical Subject Headings (MeSH) terms used for the search included combinations of “microbial contamination,” “bacterial contamination,” “viral contamination,” “parasite contamination,” and “fungal contamination” paired with descriptors such as “currency,” “banknotes,” “coins,” “paper money,” “fomite,” and “money.” The final search string was (“microbial contamination” OR “bacterial contamination” OR “viral contamination” OR “fungal contamination” OR parasite contamination”) AND (“currency” OR “banknotes” OR “coins” OR “paper money” OR “money” or “fomite”). Studies were included regardless of their design. Articles were restricted to the English language. In addition to direct searches, citations of identified studies and database-suggested similar articles were screened to ensure completeness. Gray literature, including conference abstracts, theses, and unpublished data, was excluded to prioritize peer-reviewed sources. The study selection process was documented, with all steps recorded in a PRISMA flow diagram (Figure 1).

Figure 1.

Number of records identified, screened, excluded, and included at each stage of the systematic review, following PRISMA guidelines.

Study Selection

All records retrieved from the systematic search were imported into Rayyan software ²² for efficient management and screening. Two independent reviewers evaluated the records using predefined eligibility criteria to ensure consistency and minimize bias. Studies were included if they provided qualitative or quantitative data on the microbial contamination of money, including banknotes, coins, and other currency types. The eligible study designs included cross-sectional, longitudinal, and prospective studies. Only full-text articles published in English were considered. The publications excluded from the review included reports, case-control studies, commentaries, letters to editors, preprints, review articles, and textbooks. Additionally, studies that failed to specify the sample size of the currency examined and did not report the number of positive samples were excluded. A positive sample was defined as any detectable microbial growth (bacterial, fungal, or parasitic), regardless of colony forming unit count. Studies that considered only 1 organism were excluded to avoid selection bias and ensure broader comparability across studies assessing general microbial contamination. Articles without primary data or those investigating subjects unrelated to money were also omitted. The process was carefully documented to ensure transparency. The Zotero reference management tool (Version 6.0.30, Corporation for Digital Scholarship) was used to organize references and facilitate access to full-text articles. Discrepancies between the reviewers were resolved through consultation with 2 senior authors (ESD and PBTQ), ensuring a consensus-based approach to selection.

Data Extraction

A standardized data extraction form was created for this study in Microsoft Excel 2013. Two reviewers (P.O.A. and A.O.) independently extracted data from the eligible studies for review. The key data points extracted from each study included the following: citation details, type of currency, microbial species isolated, number of currency samples collected and those testing positive, percentage positivity, sampling techniques, microbial identification methods, culture media, and broth used, incubation conditions, geographic region, additional data type of currency (coins or paper) and appearance of currency (very dirty, dirty, and clean). The percentages were standardized as whole numbers to ensure consistency across datasets. All disagreements were resolved through discussion. A third reviewer (E.S.D.) also confirmed the extracted data in an attempt to minimize bias.

Quality Assessment

The quality of the included studies was assessed via the 18-item Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist. ²³ Two independent reviewers (POA and AO) conducted the evaluation, and any disagreements were resolved through discussion and, where necessary, by consulting a senior author (PBTQ).

The key domains from the STROBE checklist used for the assessment included the following:

• Title and Abstract: Clarity and coherence in presenting the study objectives, background, and summary of findings.

• Introduction: Justification of the study, with a clear explanation of the research rationale and objectives.

• Methods: Appropriateness of the study design, description of the setting, participant recruitment, data sources and measurements, handling of variables, mitigation of potential biases, determination of the study size, and use of statistical methods.

• Results: Through the reporting of descriptive and outcome data, the results were presented in a transparent and logical manner.

• Discussion: Interpretation of key findings, acknowledgment of limitations, and consideration of the generalizability of the results.

• Other Information: Disclosure of funding sources and any conflicts of interest.

Each study was scored on the basis of its adherence to these criteria, with quality scores ranging from 9 to 14. The scoring system for quality appraisal and the assessment of the included studies are provided in Supplemental Document 1.

Data Synthesis and Meta-Analysis

A random-effects meta-analysis was performed to pool the prevalence estimates of microbes (bacteria, parasites and fungi) on money. Subgroup analyses were conducted on the basis of factors such as microbial type (bacteria, fungi and parasites), currency type (banknotes vs coins), and money condition/appearance (old or visibly dirty and moderately dirty). Heterogeneity was assessed via the I² statistic, and potential sources of heterogeneity were explored via subgroup analyses and meta-regression. Publication bias was evaluated through visual inspection of funnel plots and statistical tests, such as Egger’s test. The meta-analysis was conducted via R version 4.3.3. Statistical significance was set at a two-tailed P-value of less than .05. The dataset used for the meta-analysis is presented in Supplemental Document 1.

Results

Search Results

A total of 1078 records were identified through database searches (PubMed, Web of Science, Google Scholar, and Scopus), with an additional 31 records obtained through citation and related-article searches. After removing 414 duplicates, 664 unique records were screened by title and abstract, resulting in the exclusion of 588 that did not meet the inclusion criteria. Of the 76 full-text articles retrieved, 13 could not be accessed or were deemed irrelevant. The remaining 63 articles were assessed in detail, and 28 were excluded for reasons such as inadequate data, poor representativeness, or limited relevance. In total, 35 studies met the eligibility criteria and were included in the systematic review (Figure 1).

Study Characteristics

A total of 35 studies^{1,2,5 -7,13 -18,24 -46} from 14 countries were included in this review (Table 1). The geographic distribution of the studies reflects a global concern regarding microbial contamination of money, with the majority of the studies conducted in developing countries. Nigeria contributed the greatest number of studies (13), followed by India (6). Other countries represented include Ghana, Iraq, Egypt, Pakistan, Brazil, Poland, the United States, Yemen, Libya, Cameroon, Mexico, and Iran. ¹¹ The present study focused exclusively on bacterial contamination. Three studies investigated fungal contamination alone,^6,34,37 whereas 13 studies examined parasitic contamination exclusively.^{2,5,13,32,38,40 -42,44 -48} Two studies reported both bacterial and parasitic contamination,^28,43 and 6 studies investigated both bacterial and fungal contamination.^{15,18,26,33,35,36} Although viral contamination was included in the search strategy, no eligible studies assessing viral presence in currency were identified. Five studies provided distinct information on the microbial contamination of paper money and coins. Two studies also provided distinct information on microbial contamination on the basis of the condition or appearance of the money. The studies utilized samples from a diverse range of environments and occupational groups, including markets, hospitals, grocery stores, butchers, food vendors, public transport systems, beggars, bus conductors, and roadside mechanics. Sample sizes vary widely, ranging from as few as 30 samples ²⁹ in India to as many as 519 samples ³⁸ in Iran. Paper currency was the predominant focus, with denominations ranging from low-value notes such as ₦5 (Nigeria) and INR 10 (India) to higher-value denominations such as the 100 000 Iranian Rials and 1000 Yemeni Rials. Five studies categorized currency on the basis of its physical condition, identifying notes as new, moderately used, or old. The full details of the study characteristics are provided in Supplemental Document 7.

Table 1.

Pooled Prevalence of Individual Fungal Species Isolated from Contaminated Currency.

Organism	Prevalence (95% CI)	I 2	P-value	Number of studies
Absidia species	0.5% (0.0-0.26)	N/A	N/A	1
Agglomerans	3.3% (0.4-11.5)	N/A	N/A	1
Alternaria tenuis	18.3% (9.5-30.4)	N/A	N/A	1
Aspergillus flavus	32.1% (29.1-60.2)	91%	<.01	2
Aspergillus fumigatus	32% (19.5-46.7)	N/A	N/A	1
Aspergillus niger	42.2% (29.1-55.3)	75%	.02	3
Aspergillus parasiticus	5.0% (1.0-13.9)	N/A	N/A	1
Aspergillus species	17.4% (12.5-23.1)	N/A	N/A	1
Blastomyces species	0.9% (0.1-3.4)	N/A	N/A	1
Candida albicans	3.3% (1.3-6.7)	N/A	N/A	1
Candida glabrata	26.0% (14.6-40.3)	N/A	N/A	1
Candida species	4.7% (0.0-12.5)	93%	<.01	2
Epidermophyton species	2.3% (0.8-5.4)	N/A	N/A	1
Fusarium species	11.1% (0.0-25.3)	91%	<.01	3
Microsporum species	1.4% (0.3-4.1)	N/A	N/A	1
Mucor species	36% (28.3-43.7)	0	1.00	2
Penicillium species	26% (13.9-38.0)	81%	<.01	3
Rhinosporidiosis species	22% (11.5-36.0)	N/A	N/A	1
Rhizomucor species	22% (11.5-36.0)	N/A	N/A	1
Rhizopus	27.2% (0.0-63.5)	98%	<.01	2
Saccharomyces cerevisiae	10.8% (7.0-15.8)	N/A	N/A	1
Scopulariopsis species	0.5% (0.0-2.6)	N/A	N/A	1
Sporortichum species	1.7% (0.0-8.9)	N/A	N/A	1
Sporothrix species	0.9% (0.1-3.4)	N/A	N/A	1
Trichoderma viride	13.6% (5.5-21.7)	42%	.19	2
Trichophyton species	8.5% (5.1-13.0)	N/A	N/A	1
Yeast	15% (8.6-23.5)	N/A	N/A	1

Microbial Contamination of Money: Pooled Prevalence and Distribution

The meta-analysis revealed that 12.8% (95% CI: 10.9-14.7) of the examined money samples carried microbial (bacteria, fungi and parasites) contaminants. Among the identified contaminants, bacteria 14.0% (95% CI: 11.1-16.8), fungi 17.0% (95% CI: 12.1-21.8), and parasites 9.7% (95% CI: 6.8-12.7) were frequently isolated (Figures 2-4).

Figure 2.

Pooled prevalence of bacterial contamination.

Figure 3.

Pooled prevalence of fungal contamination.

Figure 4.

Pooled prevalence of parasitic contamination.

Prevalence of Individual Fungal Species Isolated from Contaminated Currency

Fungal contamination was reported in 9 studies, with 27 unique fungal taxa identified (Table 1). Among the fungal organisms reported in currency contamination studies, Aspergillus niger was the most frequently detected, identified in 3 independent studies with a high prevalence of 42.2% (95% CI: 29.1-55.3); that is, the proportion of contaminated currency samples testing positive for this organism. This was followed by Penicillium species 26% (95% CI: 13.9-38.0) and Fusarium species 11.1% (95% CI: 0.0-25.3), indicating the recurring presence of environmental filamentous fungi. Opportunistic pathogens such as Candida glabrata 26.0% (95% CI: 14.6-40.3) and Rhizopus spp. 27.2% (95% CI: 0.0-63.5) were noted. The meta-analysis plots for the pooled prevalence of fungal contamination are provided in Supplemental Document 6.

Prevalence of Individual Bacterial Species Isolated from Contaminated Currency

Bacterial contamination was reported in 19 studies, identifying over 40 distinct bacterial organisms (Table 2). Escherichia coli, a key fecal indicator organism, was isolated in 13 studies, with a pooled prevalence of 16.8% (95% CI: 5.1-28.6). The most prevalent bacterial species were coagulase-negative Staphylococcus 29.9% (95% CI: 12.7-26.8), Klebsiella spp. 20.3% (95% CI: 10.4-30.2), and Staphylococcus aureus 19.8% (95% CI: 12.7-26.8). Several other clinically relevant bacteria, including Enterobacter spp. 5.8% (95% CI: 3.1-8.5), Shigella spp. 18.1% (95% CI: 0.0-44.1), and Salmonella spp. 5.7% (95% CI: 0.0-11.4), were less frequently reported. While some species, such as Serratia 45% (95% CI: 35.0-55.3) and Bacillus cereus 28% (95% CI: 19.5-37.9), were reported in only 1 study, their high contamination rates suggest localized outbreaks or methodological differences. The meta-analysis plots for the pooled prevalence of bacterial contamination are provided in Supplemental Document 6.

Table 2.

Pooled Prevalence of Individual Bacterial Species Isolated from Contaminated Currency.

Organism	Prevalence (95% CI)	I 2	P-value	Number of studies
Acinetobacter species	5.4% (0.0-11.7)	83%	<.01	3
Bacillus cereus	28.0% (19.5-37.9)	N/A	N/A	1
Bacillus species	19.6% (0.8-38.3)	98%	<.01	4
Citrobacter species	11.5% (6.6-18.2)	N/A	N/A	1
Citrobacter diversus	3.3% (0.4-11.5)	N/A	N/A	1
Citrobacter freundii	1.7% (0.0-8.9)	N/A	N/A	1
Coagulase-negative Staphylococcus	29.9% (14.0-45.8)	98%	<.01	7
Corynebacterium spp.	8.5% (5.1-13.0)	N/A	N/A	1
Enterobacter aerogenes	0.9% (0.1-3.4)	N/A	N/A	1
Enterobacter cloacae	13.3% (5.9-24.6)	N/A	N/A	1
Enterobacter sakazakii	10% (3.8-20.5)	N/A	N/A	1
Enterobacter species	5.8% (3.1-8.5)	0	.51	3
Enterococcus faecalis	10% (4.9-17.6)	N/A	N/A	1
Enterococcus	4.9% (0.0-14.6)	85%	<.01	2
Escherichia coli	16.8% (5.1-28.6)	96%	<.01	13
Haemophilus influenza	20.0% (12.7-29.2)	N/A	N/A	1
Klebsiella ozaenae	1.7% (0.0-8.9)	N/A	N/A	1
Klebsiella species	20.3% (10.4-30.2)	96%	<.01	9
Klebsiella−Aerobacter	4% (1.5-8.5)	N/A	N/A	1
Listeria monocytogenes	5% (1.6-11.3)	N/A	N/A	1
Micrococcus	7.5% (2.7-12.4)	74%	.05	2
Pneumoniae	11% (5.6-18.8)	N/A	N/A	1
Proteus mirabilis	1.7% (0.2-3.2)	65%	.02	5
Pseudomonas aeruginosa	8.6% (4.4-12.8)	87%	<.01	9
Nonfermentors	3.2% (0.0-7.7)	58%	.12	2
Pantoea species	0.8% (0.0-4.2)	N/A	N/A	1
S. pyogenes	3% (0.6-8.5)	N/A	N/A	1
Salmonella species	5.7% (0.0-11.4)	87%	<.01	3
Serratia species	45% (35.0-55.3)	N/A	N/A	1
Shigella dysenteriae	3.3% (1.3-6.7)	N/A	N/A	1
Shigella species	18.1% (0.0-44.1)	97%	<.01	3
Staphylococcus species	21.2% (7.3-35.0)	98%	<.01	5
Staphylococcus aureus	19.8% (12.7-26.8)	86%	<.01	7
Staphylococcus coagulase−positive	14.7% (9.4-21.4)	N/A	N/A	1
Streptococcus species	9% (0.0-22.4)	96%	<.01	2
Unidentified bacteria	20.0% (14.6-25.4)	0%	.71	2

Prevalence of Individual Parasitic Species Isolated from Contaminated Currencies

Parasites were reported in 15 studies, with Ascaris lumbricoides 15.7% (95% CI: 4.9-26.5) and Entamoeba histolytica 18.9% (95% CI: 1.3-36.5) being the most frequently encountered parasites identified in 14 and 11 studies, respectively (Table 3). Other commonly reported parasites included Trichuris trichiura 9.3% (95% CI: 2.1-16.6), Enterobius vermicularis 6.3% (95% CI: 1.8-10.8), and Giardia lamblia 6.9% (95% CI: 0.3-13.6). Notably, hookworms were detected in 12 studies, albeit with a slightly lower pooled prevalence of 5.8% (95% CI: 3.7-8.0). Although some species, such as Balantidium coli 34% (95% CI: 25.0-43.8), have been reported in single studies, their presence indicates localized sanitary conditions and the potential for money to mediate fecal-oral transmission. The meta-analysis plots for the pooled prevalence of parasitic contamination are provided in Supplemental Document 6.

Table 3.

Pooled Prevalence of Individual Parasitic Species Isolated from Contaminated Currency.

Organism	Prevalence (95% CI)	I 2	P-value	Number of studies
Ascaris lumbricoides	15.7% (4.9-26.5)	98%	<.01	14
Balantidium coli	34.0% (25.0-43.8)	N/A	N/A	1
Cryptosporidium parvum	3.9% (2.4-5.9)	N/A	N/A	1
Endolimax nana	15.9% (9.0-25.2)	N/A	N/A	1
Entamoeba coli	13.9% (0.0-32.9)	96%	<.01	3
Entamoeba histolytica	18.9% (1.3-36.5)	99%	<.01	11
Enterobius vermicularis	6.3% (1.8-10.8)	91%	<.01	8
Fasciola hepatica	2.3% (1.2-4.0)	N/A	N/A	1
Flagellates	3.8% (0.4-7.3)	78%	<.01	4
Fluke	1% (0.1-2.7)	N/A	N/A	1
Freeliving nematode	0.7% (0.1-2.7)	N/A	N/A	1
Giardia lamblia	6.9% (0.3-13.6)	86%	<.01	5
Hookworm	5.8% (3.7-8.0)	81%	<.01	12
Hymenolepis nana	11.6% (0.0-30.7)	95%	<.01	2
Isospora belli	2.2% (0.0-4.6)	69%	.04	3
Lice	2.8% (0.0-6.6)	81%	.02	2
Strongyloides stercoralis	4.2% (0.0-9.1)	81%	<.01	3
Taenia species	2.6% (0.0-7.0)	89%	<.01	3
Toxocara cati	1.5% (0.7-3.0)	N/A	N/A	1
Trichuris trichiura	9.3% (2.1-16.6)	91%	<.01	6
Unidentified parasite	8.3% (1.2-15.4)	88%	<.01	5

Microbial Contamination of Money by Currency Type and Physical Condition

Currency type was found to influence the degree of microbial contamination. Paper money presented a higher pooled contamination prevalence 21.3% (95% CI: 13.0-29.6) than did coins 14.8% (95% CI: 4.3-25.3) on the basis of 5 studies each (Table 4). When stratified by microbial group, bacteria were the most frequently identified pathogens on both money types, with a slightly higher pooled prevalence on coins 26.4% (95% CI: 8.6-44.2) than on paper notes 23.7% (95% CI: 9.5-37.8). Parasitic contamination was substantially more prevalent on paper currency (18.1%; 2 studies) than in coins 2.1% (95% CI: 1.2-3.0).

Table 4.

Microbial Contamination of Money by Currency Type and Physical Condition.

Microbial contamination by money type	Prevalence (95% CI)	I2 (%)	P-value	Number of studies
Coins	14.8% (4.3-25.3)	96	<.01	5
Paper	21.3% (13-29.6)	92	<.01	5
Microbial types on coins
Bacteria	26.4% (8.6-44.2)	97	<.01	3
Parasite	2.1% (1.2-3.0)	47	.07	2
Microbial types on paper money
Bacteria	23.7% (9.5-37.8)	95	<.01	3
Parasite	18.1%	76	<.01	2
Microbial contamination based on appearance
Old/very dirty money	3.0% (2.3-4.1)	0	.59	2
Moderate dirty money	2.2% (1.3-3.7)	0	.98	2

The relationship between the physical condition of money and its microbial burden was assessed in 2 studies. Currencies classified as old or visibly dirty had a slightly higher pooled contamination prevalence of 3.0% (95% CI: 2.3-4.1) than did moderately dirty currency, which had a pooled prevalence of 2.2% (95% CI: 1.3-3.7). Although the differences were modest and heterogeneity was negligible, these results may support the hypothesis that heavily circulated or visibly degraded currency accumulates greater microbial load, possibly due to increased surface roughness, loss of protective coatings, or prolonged environmental exposure. The meta-analysis plots for the pooled prevalence based on the condition of money, contamination by money type, contamination on paper money and contamination on coins are provided in Supplemental Documents 2-5, respectively.

Meta-Regression and Publication Bias

The regression model identifies 2 key variables, “Moderate/Dirty” and “Old/Very Dirty,” that have statistically significant effects (P < .05) on contamination levels. Other variables, such as Coins, Fungi, Paper, and Parasite, were not significantly associated with contamination level (Table 5). Egger’s test of funnel plot asymmetry revealed statistically significant evidence of small-study effects or publication bias (t = 2.69, df = 283, P = .0075). The positive bias estimate (0.7997) suggests that smaller studies tend to report higher contamination levels than larger studies do. Figure 5 presents the funnel plot of the meta-analysis, illustrating the distribution of standard error against the proportion of microbial contamination.

Table 5.

Regression Analysis of Factors Associated with Currency Contamination Levels.

Variable	Estimate	Standard error (SE)	z-Value	P-value	Lower CI	Upper CI
Coins	−5.0961	4.3072	−1.1832	.2367	−13.538	3.3459
Fungi	−2.4605	3.1079	−0.7917	.4285	−8.5519	3.6309
Moderate/dirty	−16.2511	5.2158	−3.1157	.0018	−26.4739	−6.0282
Old/very dirty	−14.8065	4.4925	−3.2958	.001	−23.6116	−6.0014
Paper	−6.5246	3.6865	−1.7699	.0767	−13.75	0.7007
Parasite	−1.3532	2.5654	−0.5275	.5979	−6.3813	3.6749

Figure 5.

A funnel plot of the meta-analysis showing the standard error and proportion of microbial contamination.

Discussion

Money is one of the most circulated items within human society and passes through thousands of hands daily in economic, health care, and food interactions. ¹ This study also confirmed that currency can be an important reservoir for microbial contamination, which has also been reported in other studies.^11,49 Fungi presented the highest contamination burden, followed by bacteria and parasites. This reflects a common limitation in the literature, where many studies have historically underrepresented fungi and parasites, often owing to limited detection methods, lack of routine fungal culture, or a stronger public health focus on bacteria. This is an important public health concern, particularly in low- and middle-income nations, where cash remains the most common means of financial transaction and sanitation practices are also often very poor.^28,50 Since money is rarely disinfected or washed away, the persistence of contamination observed in this study implies a significant and ongoing risk of indirect transmission of disease even though there is no direct evidence for the presence of pathogenic microorganisms over currency, which results in infection. ⁴⁹ The presence of potentially pathogenic organisms on money in this review should therefore be interpreted as an indicator of exposure risk rather than conclusive evidence of disease causation. Importantly, while many studies have confirmed the presence of viable organisms, the reported microbial loads are generally low and may fall below the infectious threshold under ordinary handling conditions. No eligible studies on viral contamination of currency were identified. This indicates a critical gap in the literature because many viruses can persist on inanimate surfaces and remain infectious for extended periods.^51,52 Future research should therefore consider investigating the persistence, viability, and potential infectivity of viruses in different currencies.

The extent of bacterial contamination detected in the banknotes was high, with Escherichia coli, Staphylococcus aureus, and Klebsiella spp. being the most prevalent species. The detection of E. coli, Shigella, and Salmonella has been widely accepted as an indicator of fecal contamination, which may be attributed to the possibility that many people disregard hand washing after using toilets, indicating that paper currencies can act as potential sources of enteric diseases, as also suggested by Sunil et al ¹⁷ and Sharma and Sumbali. ⁴⁹ Considering that these isolated bacterial species are the key etiological agents of various infections, their presence on money suggests a potential exposure risk through fomites, particularly in settings with high contact intensity, such as food courts and public transportation networks. ⁴¹ In the last 30 years, the frequency, etiology, and epidemiology of bloodstream infections (BSIs) have changed with the evolution of medical care, particularly among the increasing number of hospitalized patients who require intensive care. Currently, the 3 most common causes of nosocomial BSIs in the United States are coagulase-negative staphylococci, Staphylococcus aureus, and enterococci. ⁵³ The continuous recovery of these pathogens in addition to other pathogens, such as Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumannii, and Enterobacter spp., which are closely associated with hospital-acquired and bloodstream infections and antimicrobial resistance,^{53 -55} confirms the findings of previous studies suggesting that banknotes can act as a source for both community and nosocomial infections, particularly in medical settings and high-contact settings. ⁵⁶

This review also shows that money is frequently contaminated with a variety of fungal species, including several of medical importance. The fungi identified included B. dermatitidis, A. niger, A. fumigatus, Fusarium spp., Mucor spp., Penicillium spp., C. albicans, Rhizopus spp., Trichoderma reesei, Colletotrichum gloeosporioides, and Colletotrichum truncatum. Among these, Aspergillus niger was the most frequently isolated fungus across the included studies. Some authors^36,57 have similarly reported such findings. Fungal contamination, specifically the significant presence of potential pathogenic fungi, is important in view of the durability displayed by fungal spores in hostile environments. ⁵⁸ At particular stages during their life cycles, fungi use multiple strategies to form specialized structures to survive unfavorable environmental conditions. These strategies encompass sporulation as well as cell wall melanization, multicellular tissue formation, or even dimorphism. ⁵⁹ The resulting structures are not only used to disperse to other environments but also to survive long periods of time awaiting favorable growth conditions, unlike bacteria, whose survival is contingent on moisture.^59,60 Therefore, in terms of adaptability, they are best suited for currency, which is not regularly cleaned and disinfected. ⁶¹ The presence of Aspergillus species on money is especially concerning for individuals with weakened immune systems. Exposure to species such as A. niger and A. fumigatus can lead to serious infections such as pulmonary aspergillosis, which may become life-threatening.^62,63 Similarly, Candida species cause candidaemia, a key etiological cause of fungal bloodstream infection in a hospital environment.^{64 -67} These findings suggest that contaminated currency can serve as a possibly under recognized vector for fungal infection in both healthcare and community environments.

Although parasitic contamination is less common than bacterial and fungal contamination is, the high prevalence of Ascaris lumbricoides, Entamoeba histolytica, and Giardia lamblia remains a major concern. This likely reflects poor hygiene practices and unsafe methods of handling money, as well as limited awareness of the health risks associated with contaminated currency. Parasite cysts and eggs isolated from money are highly risky to humans. Most of the isolated parasites can be transmitted via the oral route. This becomes more worrisome when it is considered in underdeveloped countries, where many people tend to wet their fingers when counting money. ⁶⁸ A. lumbricoides causes ascariasis, a disease that spreads through oral contact with materials contaminated with the ova of Ascaris. ⁶⁹ Ascaris eggs are quite resilient and can survive extreme environmental conditions. The eggs are coated with mucopolyssacaride, which makes them adhesive to surfaces, including paper currencies.^{68 -70} Infection with a large number of Ascaris worms can lead to abdominal pain or even intestinal obstruction. ⁷¹ E. histolytica causes amebiasis, a disease transmitted through ingestion of the parasite’s cyst. The pathology of the invasive form of amebiasis manifests as amoebic dysentery, liver abscess, or possible death. ⁷²

A noteworthy observation was the high level of banknote contamination compared with that of coins. Compared with metal, paper banknotes, consisting of linen and cotton fibers, form a particularly conducive porous substrate for microbe adhesion, allowing for extended survival of parasites, fungi, and bacteria.^18,56 In contrast, metal coins, with a high proportion of nickel and copper, have inbuilt antimicrobial properties that inhibit microbial viability. ³¹ The finding that older banknotes, which are visibly contaminated, have a high microbial burden is in agreement with studies suggesting that banknotes circulated for extended periods build organic matter and microbial biofilms and, consequently, become increasingly effective at disease transmission.^49,73

The findings of such studies have important implications for public health, particularly in low- and middle-income nations (LMICs), in view of the widespread use of cash payments in such countries due to limited access to electronic payment infrastructure. ⁷⁴ The presence of pathogenic microbial species plays a critical role in focused hygiene interventions in settings with increased risk, including food preparation, healthcare settings, and public transportation networks. According to the U.S. Federal Reserve, the lifespan of money varies by denomination and depends on several factors, including how the denomination is used by the public. For example, larger denominations such as $100 notes are often used as a store of value and have a life span of 22.9 years, which means that they pass between users less frequently than lower denominations such as $5 notes with a life span of 4.7 years, which are more often used for transactions. ⁷⁵ This long duration increases the risk of transmitting pathogens among individuals who handle cash regularly. Moreover, the majority of banknotes used worldwide are made of paper,^56,76,77 making them difficult to clean without being damaged. ⁵⁶ To alleviate such risks, public health programs must prioritize raising awareness about hand washing, promoting contactless payments, and studying the viability of antimicrobial coatings for banknotes. Countries that have adopted polymer banknotes, such as Canada and Australia, have seen a reduced level of contamination. ⁷⁷ This may suggest that a switch to alternative, non-cotton banknotes could represent a viable long-term intervention. Intervention studies have become a key target for testing the effectiveness of UV disinfection, antimicrobial coatings, and routine banknote sanitization.

Meta-regression identified currency condition as a key determinant of contamination, with moderately dirty and very dirty/old notes showing significantly higher microbial loads (P < .01). Other factors currency type and microbial group were not significant. This may suggest that surface wear and circulation duration outweigh material in predicting contamination. Pooled estimates exhibited very high heterogeneity (I² > 90%). This may indicate substantial between-study variability. Because of this pooled prevalence should be interpreted cautiously and viewed as a broad summary rather than a precise point estimate. Egger’s test showed evidence of small-study effects (P = .0075), with smaller studies reporting higher contamination, which may reflect methodological differences or selective reporting.

Limitations

The studies included varied widely in design, laboratory techniques, and reporting quality. Pathogen identification methods range from highly sensitive molecular assays to conventional culture-based techniques, which likely influence both the detection rates and the spectra of the organisms identified. Many studies have assessed only bacterial contamination, with fewer examining fungi or parasites, leading to potential underrepresentation of nonbacterial pathogens. In addition, key variables such as currency denomination, material type, frequency of use, and handling practices were often missing or inconsistently reported, limiting the depth of subgroup analyses. Environmental and regional differences, including climate, sanitation infrastructure, and hygiene behaviors, also influence microbial survival, yet these factors are rarely accounted for in study designs. While subgroup analyses by currency type, organism group, and physical condition were performed to reduce this variation, limited reporting on contextual variables prevented deeper stratified analysis. The review also relied on English-language publications and excluded gray literature, which may have introduced language or publication bias. Despite these limitations, this review provides a clearer understanding of the global extent of microbial contamination and highlights the need for standardized, high-quality studies to support effective public health action.

Conclusion, Recommendations, and Future Directions

This review demonstrates that money frequently harbors bacteria, fungi, and parasites, with paper banknotes showing higher contamination levels than coins do. The predominance of enteric and opportunistic pathogens suggests that currency can serve as a potential vehicle for microbial exposure, even though direct infection through handling money remains difficult to quantify. The findings of this review make 1 point undeniably clear: money moves, and sodo microbes. As such, public health policies and personal habits must evolve to reduce the risk of currency-borne transmission.

We offer the following evidence-informed recommendations and hypotheses for future studies:

• Promote hand hygiene as a universal practice. Alcohol-based hand sanitizers and frequent handwashing after handling money, particularly before meals or food preparation, are widely encouraged. These simple habits remain among the most effective ways to prevent fomite-mediated infections.

• End the practice of wetting fingers with saliva to count money. Common in many markets, transport hubs, and informal businesses, this habit creates a direct pathway for pathogens into the body. Public health messaging should actively target this practice as avoidable and unsafe.

• Discourage children from handling currency. Across many households, particularly across Africa, it is common for children to be given money to play with. This poses a silent but serious risk, as young children may place contaminated notes or coins in their mouths, exposing themselves to parasites, bacteria, and fungi.

• Support a transition to electronic and contactless payment systems. Where digital infrastructure exists, governments and institutions should promote mobile and contactless payments, particularly in high-contact environments such as healthcare, food service, and public transport.

• Encourage central banks to adopt polymer banknotes or antimicrobial coatings. Compared with traditional cotton-based notes, polymer currency has demonstrated lower rates of microbial contamination and is easier to disinfect. The incorporation of antimicrobial materials or treatments during currency production could further reduce contamination.

• Replace old and visibly dirty notes more frequently. Paper money that is torn, stained, or excessively worn harbors significantly more microbial matter. Central banks and financial institutions should establish systems to remove such notes from circulation promptly.

• Currency hygiene should be integrated into public health education. Awareness campaigns should highlight that while money is essential, it can act as a microbial reservoir. Public education should promote safe handling practices without inducing fear or stigma.