Environmental Carcinogens: Testing and Managing Risks

Every day, people are exposed to substances in the air they breathe, the water they drink, and the soil beneath their feet – some of which have the potential to cause cancer. These substances, known as environmental carcinogens, are a serious public health concern. But how do scientists actually test whether a chemical is carcinogenic? And what strategies are in place to manage these risks? This post breaks down the main sources of environmental carcinogens, the testing methods used to evaluate their cancer-causing potential, and the risk assessment frameworks that guide protective action.

What are environmental carcinogens? 🔗

Environmental carcinogens are agents found in the natural or human-altered environment that can trigger the development of cancer. They are broadly grouped into three categories: physical carcinogens (like ionizing radiation and UV rays), chemical carcinogens (such as polycyclic aromatic hydrocarbons, benzene, and asbestos), and biological carcinogens (including certain viruses like HPV and hepatitis B). These agents interact with cellular DNA in different ways – physical agents tend to cause strand breaks, chemical agents form DNA adducts, and biological agents often act through chronic inflammation or insertional mutagenesis.

What makes environmental carcinogens particularly challenging is that exposure is often involuntary, widespread, and occurs at low doses over long periods. According to the World Cancer Report, air pollution alone was responsible for an estimated 350,167 lung cancer deaths worldwide in 2017. The sheer number of people exposed to relatively low levels of these agents can still result in a significant number of excess cancer cases at the population level.

Sources of environmental carcinogens in air, water, and soil 🔗

Air pollution 🔗

Outdoor air is one of the most significant and well-studied sources of environmental carcinogens. The primary concern is fine particulate matter (PM2.5), a complex mixture of pollutants produced by fuel combustion in vehicles, power plants, industrial facilities, and biomass burning. Several specific agents in outdoor air have been classified as carcinogenic to humans, including benzene, diesel engine exhaust, benzo[a]pyrene, chromium, arsenic, and all forms of asbestos.

Indoor air pollution is equally important. Sources include second-hand tobacco smoke, smoke from cooking with biomass fuels, and emissions from building materials. The World Cancer Report highlights that the burden of lung cancer linked to second-hand tobacco smoke was still increasing as of 2017, reaching nearly 100,000 deaths globally that year.

Polycyclic aromatic hydrocarbons (PAHs) 🔗

Polycyclic aromatic hydrocarbons are a group of over 100 chemicals formed during the incomplete burning of coal, oil, gas, wood, garbage, and organic matter like tobacco and charbroiled meat. They are found throughout the environment – in air, water, and soil. PAHs enter the atmosphere from sources such as volcanoes, forest fires, vehicle exhaust, and industrial emissions. Food is actually a major exposure route for the general population, especially through grilled, smoked, or charred meats.

In water, PAHs typically originate from surface runoff (e.g., erosion of asphalt pavement) and industrial effluents, though their low water solubility keeps concentrations relatively low. In soil, PAHs bind tightly to particles and are relatively immobile, but they can contaminate underground water supplies. The Agency for Toxic Substances and Disease Registry (ATSDR) notes that PAHs have been identified at more than 600 of the 1,430 National Priorities List hazardous waste sites in the United States. Fifteen individual PAHs are listed by the National Toxicology Program as reasonably anticipated to be human carcinogens.

Asbestos 🔗

Asbestos is a group of naturally occurring minerals widely used in construction and insulation due to their heat resistance and durability. However, when asbestos fibres become airborne and are inhaled, they can lodge in lung tissue and lead to asbestosis, lung cancer, and mesothelioma – a rare and aggressive cancer of the chest or abdominal lining. Asbestos contaminates air, water, and soil through construction, demolition, mining, and the degradation of asbestos-containing materials.

Naturally occurring asbestos in soil has been reported in various geographical regions, and asbestos-cement water distribution pipes remain a concern for drinking water contamination. The World Health Organization has stated that there is no safe threshold for asbestos exposure – even very low levels have been associated with elevated cancer risk.

Other key carcinogens in water and soil 🔗

Arsenic in drinking water represents one of the strongest evidence-based links between a water contaminant and cancer risk. Numerous epidemiological studies have tied arsenic exposure to bladder, lung, and skin cancers. Additionally, water disinfection by-products have been associated with bladder cancer risk. In soil, the main carcinogenic contaminants include heavy metals, mineral oils, and aromatic hydrocarbons, primarily from industrial activities and waste disposal. Industrial activities account for roughly two-thirds of soil contamination across the EU, according to the European Joint Research Centre.

Risk assessment for environmental carcinogens 🔗

Determining whether a substance poses a cancer risk to humans – and how much risk – is the job of carcinogenic risk assessment. This is a structured, evidence-based process used by regulatory agencies around the world to guide policy and protective action.

The four-step risk assessment framework 🔗

The U.S. Environmental Protection Agency (EPA) follows a framework originally outlined by the National Academy of Sciences in 1983, which includes four steps:

Hazard identification is the first step. It determines whether a substance has the potential to cause cancer in humans. The EPA’s guidelines consider three broad data categories: human epidemiological data, results from long-term animal bioassays, and supporting evidence such as short-term genotoxicity tests, metabolic studies, and structure-activity relationships. These lines of evidence are combined into a weight-of-evidence characterization.

Dose-response assessment follows, establishing the mathematical relationship between the dose of a carcinogen and the likelihood of cancer developing. The EPA’s 2005 guidelines recognise both linear and nonlinear modes of action. For chemicals that cause cancer through direct DNA damage, linear extrapolation is used to estimate risk at low doses. For carcinogens with nonlinear modes of action, reference dose or reference concentration methods are applied instead.

Exposure assessment estimates how much of a substance people are actually exposed to and through which routes – inhalation, ingestion, or dermal contact. Risk characterization is the final step, where exposure estimates are combined with dose-response data to produce a numerical estimate of risk. This step also explicitly evaluates uncertainties, assumptions, and the strength of the evidence.

Classification systems 🔗

Several agencies classify substances based on their carcinogenic potential. The EPA’s 2005 guidelines use five narrative descriptors: “Carcinogenic to Humans,” “Likely to be Carcinogenic to Humans,” “Suggestive Evidence of Carcinogenic Potential,” “Inadequate Information to Assess Carcinogenic Potential,” and “Not Likely to be Carcinogenic to Humans.” The International Agency for Research on Cancer (IARC) uses a similar but distinct system with Groups 1 through 3, evaluating human, animal, and mechanistic evidence. The National Toxicology Program (NTP) classifies agents as either “Known to be a Human Carcinogen” or “Reasonably Anticipated to be a Human Carcinogen.”

It is worth noting that these classification systems focus on the strength of evidence that a substance can cause cancer, not on the level of risk it poses at any given exposure. A substance classified as “possibly carcinogenic” could, in certain exposure scenarios, present greater population-level risk than a “known” carcinogen.

Risk management strategies 🔗

Once a risk assessment is complete, the findings feed into risk management – the process of deciding what actions to take to reduce or eliminate carcinogenic risks. This involves a combination of regulatory controls, technological solutions, and public health measures.

Regulatory controls and exposure limits 🔗

Governments set legally enforceable exposure limits for known carcinogens. For airborne carcinogens, the EPA uses the unit risk estimate (URE), which represents the upper-bound excess lifetime cancer risk from continuous exposure at a concentration of 1 µg/m³. For ingested carcinogens, the carcinogenic potency slope (CPS) provides an upper-bound estimate of risk per mg/kg/day of oral exposure. These values guide the setting of ambient air quality standards, drinking water maximum contaminant levels, and occupational exposure limits.

Risk avoidance and reduction 🔗

The most effective risk management strategy is eliminating exposure entirely. Bans on asbestos production and use adopted by many countries – including former major producers like Brazil and Canada – are a prime example. However, banning alone is insufficient without thorough remediation of existing contamination. Substituting less hazardous materials for known carcinogens in industrial processes is another key approach.

Where elimination is not feasible, reducing exposure through engineering controls (such as improved ventilation, pollution control equipment, and water treatment systems), administrative measures (workplace rotation, restricted access zones), and personal protective equipment helps lower risk. Monitoring programs that regularly test air, water, and soil quality provide early warning of contamination spikes.

Community-level prevention 🔗

Prevention of exposure to environmental carcinogens requires both regulatory action and community engagement. Land-use planning that separates industrial zones from residential areas, soil remediation at contaminated sites, and public awareness campaigns about reducing personal exposures (such as avoiding charring food or using proper ventilation when cooking with solid fuels) all play important roles.

Bioassays for environmental carcinogenicity testing 🔗

Bioassays are the backbone of experimental carcinogenicity testing. A bioassay is an analytical method that uses living organisms – either whole animals (in vivo) or cells and tissues (in vitro) – to evaluate the toxic or carcinogenic potential of a substance.

The two-year rodent bioassay 🔗

The two-year rodent bioassay has long been considered the gold standard for carcinogenicity testing. It typically involves exposing groups of rats and mice of both sexes to different doses of a test chemical for most of their natural lifespan – about two years. According to the National Toxicology Program (NTP), these long-term studies are the primary method for identifying carcinogenic substances. At least 50 animals per dose group are used, and they are carefully monitored for tumour development compared to unexposed control groups.

The current standard approach combines genotoxicity testing with the two-year bioassay. Short-term genotoxicity tests, such as the widely used Ames test (which detects mutations in Salmonella bacteria), serve as an initial screening tool. If a substance shows mutagenic activity, it becomes a higher priority for the more resource-intensive long-term bioassay.

Short-term and alternative testing models 🔗

Because two-year rodent bioassays are slow, expensive, and raise ethical concerns, scientists have developed alternative approaches. Short-term toxicology screens – typically 14-day or 13-week studies – assess acute toxic effects and help establish appropriate dose levels for longer studies. Transgenic mouse models, such as the hemizygous p53 model, offer greater sensitivity and specificity for detecting carcinogens in a shorter timeframe. Research published in Environmental Health Perspectives has shown that these models can identify mutagenic carcinogens more efficiently and could be incorporated into the testing toolkit without replacing the conventional bioassay entirely.

Small fish species have also been used as test organisms in carcinogenicity bioassays. They can be bred in large numbers, have low maintenance costs, and show a low background incidence of tumours, making them useful both as environmental sentinel organisms and as experimental models.

In vitro approaches and the future of testing 🔗

In vitro cell transformation assays (CTAs) represent a promising frontier. These use cultured human or rodent cells exposed to test substances under controlled conditions. Researchers at various institutions have proposed using batteries of in vitro assays that cover multiple hallmarks of the carcinogenesis process – including DNA damage, oxidative stress, cell proliferation, and evasion of apoptosis – to provide a more comprehensive assessment than any single test can offer. As noted in research published in the International Journal of Molecular Sciences, long-term in vitro exposure studies (lasting six to thirty weeks) can more closely mimic the chronic, low-dose environmental exposures that humans actually face.

Additionally, the micronucleus assay – which detects chromosome damage in developing red blood cells – is an important complementary tool. Micronuclei form when chromosomes break or fail to migrate properly during cell division, and their detection is a recognised biomarker for structural DNA damage linked to cancer.

Weight-of-evidence approach 🔗

No single test can definitively prove or disprove a substance’s carcinogenicity. That is why regulatory agencies such as the EPA and IARC use a weight-of-evidence approach, combining results from molecular studies, short-term genotoxicity tests, animal bioassays, and epidemiological studies. Each method has strengths and limitations: short-term tests provide rapid screening but may miss non-mutagenic carcinogens; animal bioassays offer controlled conditions but may not perfectly predict human outcomes; and epidemiological studies examine real human populations but face challenges in isolating specific chemical effects. Together, they provide a more reliable basis for making carcinogen classification and risk management decisions.

Challenges and the road ahead 🔗

Despite decades of progress, significant challenges remain. Thousands of chemicals in commercial use have never been tested for carcinogenicity. The two-year bioassay, while rigorous, can evaluate only a limited number of substances due to its cost and duration. Regulatory gaps also exist – for instance, while airborne asbestos exposure is regulated in many countries, standards for asbestos in soil and water are far less developed.

Advances in high-throughput screening, computational toxicology, biomarker development, and exposome research are beginning to address these gaps. These newer methods can generate large datasets covering a wide range of chemicals and biological endpoints, though they are not yet fully validated for formal regulatory use. The goal is a testing ecosystem that is faster, more cost-effective, and better at predicting real-world human cancer risk.

What do you think? Given the sheer number of untested chemicals in our environment, how should regulators prioritise which substances to evaluate first? And do you think advances in in vitro and computational testing can eventually replace traditional animal bioassays, or will we always need both approaches?

References

1. https://er.researchfloor.org/role-of-environmental-carcinogens-in-dna-damage-and-cancer-initiations-pathways/
2. https://www.ncbi.nlm.nih.gov/books/NBK606470/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC11962547/
4. https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsDetails.aspx?faqid=121&toxid=25
5. https://wwwn.cdc.gov/TSP/PHS/PHS.aspx?phsid=120&toxid=25
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC10256854/
7. https://www.epa.gov/fera/risk-assessment-carcinogenic-effects
8. https://www.epa.gov/risk/guidelines-carcinogen-risk-assessment
9. https://www.atsdr.cdc.gov/pha-guidance/conducting_scientific_evaluations/indepth_toxicological_analysis/EvaluateEvidenceCancerEffects.html
10. https://ntp.niehs.nih.gov/research/apptoxres/cartox
11. https://pmc.ncbi.nlm.nih.gov/articles/PMC1519166/
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC10178670/
13. https://pmc.ncbi.nlm.nih.gov/articles/PMC12026592/