Peanuts do not contain any inherently cancer-causing substance, but they are one of the crops most vulnerable to contamination by aflatoxins, a group of toxins produced by soil-dwelling fungi that are classified as potent human carcinogens. The cancer in question is liver cancer, specifically hepatocellular carcinoma, and the risk depends heavily on how much contaminated food a person eats over time, where they live, and whether they carry hepatitis B. For someone buying peanut butter off a supermarket shelf in a country with strict food-safety enforcement, the risk is vanishingly small. For a subsistence farmer in a tropical region storing peanuts in a humid shed, it can be serious.
How Aflatoxin Gets Into Peanuts
The culprit is a group of fungi in the genus Aspergillus, primarily Aspergillus flavus and Aspergillus parasiticus. These molds are common in warm soils around the world, and they thrive in the same conditions peanuts grow in: heat and moderate humidity. The fungi can colonize peanut pods while they are still underground, penetrating through the outer shell and working their way into the seed tissue itself, where they begin producing aflatoxin.
1Physiological and Molecular Plant Pathology. Infection and colonization of peanut pods by Aspergillus parasiticus and the expression of the aflatoxin biosynthetic gene, nor-1, in infection hyphaeDrought stress during the pod-filling stage is one of the biggest risk factors for pre-harvest contamination. When the soil dries out and temperatures stay warm, the plant’s natural defenses weaken, and the fungus gains an advantage.
2Field Crops Research. Drought, pod yield, pre-harvest Aspergillus infection and aflatoxin contamination on peanut in NigerBut contamination does not stop at harvest. Poor post-harvest storage is just as dangerous and sometimes worse. Peanuts stored in warm, humid conditions with insect damage develop substantially higher aflatoxin levels. One study of stored peanuts found aflatoxin concentrations ranging from less than 1 ppb to nearly 28 ppb, with insect-damaged kernels showing much higher contamination.
3PubMed Central. Peanut Aflatoxin: Impact of Postharvest Insect Infestation and Storage SystemsThe fungi themselves are essentially everywhere in tropical and subtropical environments. You cannot grow peanuts in warm climates and avoid the fungus entirely. The question is always about managing the level of contamination, not eliminating the organism.
4PubMed Central. Aspergillus section Flavi and Aflatoxins: Occurrence, Detection, and Identification in Raw Peanuts and Peanut-Based Products Along the Supply ChainHow Aflatoxin Causes Liver Cancer
Aflatoxin B1, the most toxic form, does not simply poison liver cells. It gets metabolized by enzymes in the liver into a reactive compound that binds directly to DNA, forming what scientists call DNA adducts. These adducts can cause very specific mutations, particularly at a known hotspot on the p53 gene, which normally acts as the cell’s main brake against uncontrolled growth. When p53 is damaged at this particular spot (codon 249), the cell loses a critical safety check, and the path toward cancer opens. Laboratory studies have shown that the exact pattern of mutations depends on which liver enzymes process the toxin, but the characteristic p53 damage is a hallmark of aflatoxin exposure.
5PubMed. Aflatoxin B1-induced DNA adduct formation and p53 mutations in CYP450-expressing human liver cell linesThis mechanism is not speculative. The International Agency for Research on Cancer classifies aflatoxin B1 as a Group 1 carcinogen, the highest category, meaning there is sufficient evidence that it causes cancer in humans. The liver is overwhelmingly the target organ because that is where the toxin is activated. Aflatoxin exposure has not been convincingly linked to cancer at other sites in the same way.
The Hepatitis B Multiplier
One of the most striking findings in aflatoxin research is how dramatically the cancer risk increases when chronic hepatitis B virus (HBV) infection is present alongside aflatoxin exposure. Individually, each factor raises the risk of liver cancer. Together, the effect is multiplicative rather than merely additive. Studies using biomarkers of aflatoxin exposure in populations in China, Taiwan, and sub-Saharan Africa have provided strong evidence for this synergistic interaction.
6PubMed. Synergistic interaction between aflatoxin B1 and hepatitis B virus in hepatocarcinogenesisA meta-analysis quantified the scale of this synergy. The combined odds ratio for liver cancer from aflatoxin and hepatitis B together was estimated at 73, compared to about 11 for hepatitis B alone and about 6 for aflatoxin alone. The population-attributable risk of aflatoxin-related liver cancer was estimated at roughly 17% overall, but it climbed to about 21% in people who carried the hepatitis B virus and dropped to about 9% in those who did not.
7PubMed Central. Population attributable risk of aflatoxin-related liver cancer: systematic review and meta-analysisThis is why the aflatoxin-cancer link hits some regions so much harder than others. In places where hepatitis B vaccination is widespread and the virus is uncommon, aflatoxin exposure carries a much more modest risk on its own. In sub-Saharan Africa and parts of Southeast Asia, where hepatitis B is still endemic, aflatoxin exposure is an outsized contributor to liver cancer rates.
The Global Burden
Estimates of how many liver cancer cases aflatoxin causes each year vary depending on the modeling approach, but the numbers are large. One widely cited risk assessment estimated that roughly 25,000 to 155,000 of the 550,000 to 600,000 new liver cancer cases diagnosed worldwide each year may be attributable to aflatoxin, representing somewhere between 5% and 28% of the global total.
8PubMed Central. Global burden of aflatoxin-induced hepatocellular carcinoma: a risk assessmentA more recent review associated aflatoxin exposure with roughly 250,000 deaths annually and noted a two to fourfold higher risk of cancer in children exposed at high levels.
9PubMed Central. Aflatoxin exposure and health impacts: global burden and advances in detection technologiesThe burden is overwhelmingly concentrated in low-income countries in tropical zones. Dietary intake is the primary route of exposure, with daily intake estimates ranging from a fraction of a nanogram per kilogram of body weight in populations with regulated food supplies up to 180 ng/kg/day in some highly exposed communities. Prenatal transfer and breast milk are additional exposure routes for infants.
It is worth noting that the disease in question, hepatocellular carcinoma, is not a uniquely “peanut” cancer. Corn is actually a bigger contributor to aflatoxin exposure globally, because it is consumed in far larger quantities as a staple grain in many of the same regions where peanuts are also contaminated. Peanuts get attention partly because they are especially prone to Aspergillus infection and partly because they are a common export crop subject to international safety testing.
Regulatory Standards and Why They Differ
Countries set maximum allowable levels of total aflatoxins in food, but those limits vary widely. The European Union enforces one of the strictest standards: 4 parts per billion (ppb) for peanuts intended for direct human consumption. The United States allows 20 ppb in peanut products. The Codex Alimentarius, the international food standards body run jointly by the FAO and WHO, recommends 15 ppb.
10Agribusiness. Assessing Haitian Consumers’ Willingness to Pay a Premium for Aflatoxin‐Compliant Peanut Butter in the Informal MarketThese numbers represent a compromise between safety and practicality. A global risk assessment found that if the goal is to keep aflatoxin from adding more than one extra liver cancer case per 100,000 people, most current standards are not strict enough, especially in low-income countries where peanut and corn consumption is high and hepatitis B is common.
11PubMed Central. Global risk assessment of aflatoxins in maize and peanuts: are regulatory standards adequately protective?Stricter limits also create trade barriers. Developing countries that export peanuts sometimes struggle to meet the EU’s 4 ppb threshold, which can block their products from lucrative European markets. The tension between public health protection and economic access for farmers is real and ongoing.
Does Roasting or Cooking Reduce the Risk?
Somewhat, but not enough to make heavily contaminated peanuts safe. Roasting peanuts at high temperatures does break down a significant fraction of the aflatoxin present. In one detailed kinetic study, roasting at 200°C reduced aflatoxin levels by about 90%, while roasting at 160°C achieved about 62% reduction. The catch is that these results depend heavily on both time and temperature, and the starting contamination level matters too.
12PubMed. Kinetics of aflatoxin degradation during peanut roastingOther cooking methods also help. Pressure-cooking with salt and citric acid, deep frying, and roasting with additives all reduced aflatoxin B1 substantially in laboratory tests, using methods simple enough to be adopted at the household level.
13PubMed Central. Decontamination of aflatoxin B1 in peanuts using various cooking methodsThe researchers behind the roasting study were blunt in their conclusion: roasting alone is not enough to control aflatoxins in peanuts. If the starting level is very high, even a 90% reduction may leave dangerous residues. Processing is a useful layer of defense, not a substitute for keeping contamination low in the first place.
Informal Markets and Unregulated Products
The real danger zone for aflatoxin exposure is not the factory-sealed jar of brand-name peanut butter in a Western grocery store. It is the unregulated, informal supply chain. A study of raw peanuts and peanut butter sold at informal markets in Harare, Zimbabwe, illustrates the problem starkly. Sixty percent of raw peanut samples were contaminated, with total aflatoxin levels reaching as high as 426 ppb. Every single peanut butter sample tested positive, with levels up to 435 ppb. Ninety-five percent of the peanut butter samples exceeded Zimbabwe’s own legal limits.
14PubMed Central. Prevalence of Aflatoxin Contamination in Peanuts and Peanut Butter from an Informal Market, Harare, ZimbabweIn Côte d’Ivoire, one review reported that peanut paste had a 100% contamination rate, with aflatoxin B1 levels averaging over 4,500 ppb, hundreds of times above any regulatory limit anywhere in the world.
9PubMed Central. Aflatoxin exposure and health impacts: global burden and advances in detection technologiesThese are numbers that would be unthinkable in a regulated system. They reflect a reality where small-scale processors may not have access to testing, drying equipment, or proper storage, and where consumers have no way of knowing what they are eating. In Haiti, researchers found that informal market consumers were willing to pay a 12% premium for peanut butter verified to meet international aflatoxin standards, suggesting both awareness of the risk and demand for safer products.
10Agribusiness. Assessing Haitian Consumers’ Willingness to Pay a Premium for Aflatoxin‐Compliant Peanut Butter in the Informal MarketAcute Aflatoxin Poisoning
Cancer develops over years of chronic exposure, but aflatoxin can also kill quickly in large doses. Acute aflatoxicosis, caused by eating heavily contaminated food in a short period, leads to rapid liver failure and can be fatal. Outbreaks have been documented repeatedly, most often linked to contaminated corn rather than peanuts, in countries including Kenya, India, and Tanzania.
15PubMed Central. Incidence and mortality of acute aflatoxicosis: A systematic reviewAcute poisoning events tend to happen after bad harvests followed by poor storage, when people have limited food choices and end up eating the most visibly moldy grain or nuts. These episodes, while rare in countries with functioning food inspection systems, are a recurring public health emergency in parts of East Africa.
Biological Control on Farms
One of the more promising approaches to reducing aflatoxin at the source involves fighting the fungus with its own relatives. Researchers have identified strains of Aspergillus flavus that are naturally unable to produce aflatoxin. When these harmless strains are applied to soil during the growing season, they outcompete the toxin-producing strains for space and resources. Field trials in China found that treating soil with rice-based inoculant carrying a non-toxigenic strain reduced aflatoxin in harvested peanuts by roughly 70 to 80%, and at higher application rates, no aflatoxin was detected at all.
16PubMed Central. Application of Non-Aflatoxigenic Aspergillus flavus for the Biological Control of Aflatoxin Contamination in ChinaSimilar results have been reported from Argentina, where field trials during two growing seasons showed aflatoxin reductions between 78% and 90% in treated plots compared to untreated controls.
17PubMed. Non-aflatoxigenic Aspergillus flavus as potential biocontrol agents to reduce aflatoxin contamination in peanuts harvested in Northern ArgentinaProducts based on this competitive-exclusion principle are already commercially available in some markets, including the United States, where a product called Afla-Guard has been registered for use in peanut and corn fields. The concept is elegant: you do not try to kill the fungus, because it is everywhere. You change which strains dominate the local soil population.
Climate Change Is Expanding the Risk Zone
Aflatoxin contamination is not a static problem. As temperatures rise and weather patterns shift, regions that historically had low aflatoxin risk are seeing contamination creep northward. Research tracking aflatoxin B1 levels in Chinese peanut-growing regions found that both contamination rates and harvest-season temperatures have been increasing year by year in northern regions above 32°N latitude.
18PubMed Central. Dynamic changes and early warning of peanuts aflatoxin B1 contamination in China in the context of climate changeEurope is already seeing aflatoxin contamination in crops where it was once rarely detected, including corn grown in southern Europe. For peanuts specifically, this means the geographic footprint of high-risk growing conditions is expanding, and countries that currently assume low background contamination may need to revisit their monitoring programs.
Beyond Cancer: Effects on Child Growth
Aflatoxin’s health effects are not limited to liver cancer. In developing countries, where children may be chronically exposed from weaning onward, researchers have investigated whether aflatoxin stunts growth. The evidence here is mixed but concerning. A prospective study in rural Malawi found that children with higher blood levels of an aflatoxin biomarker at 18 months had significantly lower length, weight, and mid-upper arm circumference measurements that persisted through 30 months of age.
19Current Developments in Nutrition. Child Aflatoxin Exposure is Associated with Poor Child Growth Outcomes: A Prospective Cohort Study in Rural MalawiHowever, a birth cohort study in Dhaka, Bangladesh, found that after adjusting for confounders like poverty and dietary quality, the initial association between aflatoxin detection and stunting disappeared.
20PubMed Central. Aflatoxin exposure was not associated with childhood stunting: results from a birth cohort study in a resource-poor setting of Dhaka, BangladeshDisentangling aflatoxin’s direct effect from the broader package of malnutrition, infection, and poverty that accompanies it is genuinely difficult. Children who eat the most contaminated food also tend to be the poorest and least nourished, so separating cause from correlation remains an active area of research.
Clay That Binds Toxins in the Gut
For populations where contaminated food is hard to avoid, one pragmatic intervention has shown real promise: adding a specially processed calcium montmorillonite clay called NovaSil to the diet. The clay binds aflatoxin molecules tightly in the digestive tract, preventing them from being absorbed into the bloodstream. Animal studies showed it prevented aflatoxicosis without interfering with the absorption of vitamins or minerals.
21PubMed. Reducing human exposure to aflatoxin through the use of clay: a reviewA crossover trial in Kenya confirmed the approach works in humans. Participants who took the clay had significantly lower levels of aflatoxin metabolites in their urine compared to when they took a placebo, indicating that less of the toxin was making it into their bodies.
22PubMed Central. Evaluation of the efficacy, acceptability and palatability of calcium montmorillonite clay used to reduce aflatoxin B1 dietary exposure in a crossover study in KenyaThis is not a solution anyone would choose if clean food were available. But in communities where aflatoxin exposure is high and food alternatives are scarce, a cheap, tasteless clay additive that cuts the dose of a carcinogen reaching the liver is a meaningful public health tool. The intervention also underscores a broader point: the aflatoxin problem is not really about peanuts themselves, but about poverty, climate, infrastructure, and the gap between what food-safety regulators know and what farmers and consumers on the ground can actually achieve.