Inorganic Lead and Cancer

This page is for readers who want the detail on lead and cancer. It covers how agencies rate lead, what the studies show, where lead exposure comes from, and the workplace limits.

Lead is element 82. NTP notes that lead is not degraded in the environment, so whatever is released stays available for exposure.

Inorganic lead compounds are lead joined to something other than carbon chains. The list is long. Lead acetate, lead chloride, and lead nitrate dissolve in water. Lead chromate and lead phosphate do not dissolve well.

Organic lead compounds are a separate category. Tetraethyl lead and tetramethyl lead are the famous ones, once added to gasoline to stop engine knock. They matter here because the two categories are graded differently.

IARC — the International Agency for Research on Cancer — reviewed lead in Volume 87 of its monographs. Its conclusions were split:

• Inorganic lead compounds: probably carcinogenic to humans, Group 2A.
• Organic lead compounds: not classifiable, Group 3.

The working group added a wrinkle. Organic lead is metabolized, at least partly, into ionic lead inside the body. So to that extent, organic lead exposure carries inorganic lead's effects anyway.

The U.S. National Toxicology Program lists lead and lead compounds together as reasonably anticipated to be human carcinogens. That listing began in the Eleventh Report on Carcinogens in 2004, replacing narrower listings of lead phosphate and lead acetate that began in 1981.

Neither agency used its strongest word. Both stopped short deliberately, and the reason is worth understanding.

Lead exposure has been tied to lung, stomach, and urinary bladder cancer across different populations. The National Toxicology Program calls the lung and stomach evidence the strongest — and then describes it as consistent but weak.

The problems are in the study designs, not the findings. Most studies measured exposure crudely. They often treated a whole plant, or a whole job title, as if everyone in it got the same dose. Most did not control for other things that raise cancer risk. That includes lifestyle, and other carcinogens present at the same workplace. And most did not show that more lead, or longer exposure, meant more cancer. That dose-response link is usually what turns a suspicion into a conclusion.

So the epidemiology fits a small increase in lung or stomach cancer risk. It also fits confounding by smoking, by diet, or by arsenic exposure alongside the lead. IARC's phrase for this is limited evidence in humans. For how those grades work, see how the IARC monographs work and hazard versus risk.

In laboratory animals the picture sharpens, and the target organ changes.

Kidney tumors are the most common finding. Lead subacetate fed in the diet caused benign and malignant kidney tumors. It did so in mice and rats of both sexes. It also caused brain gliomas in rats. Lead acetate in food or water caused kidney adenomas and carcinomas in rats. It also raised virus-induced lymphocytic leukemia in mice. Lead phosphate injected under the skin caused kidney tumors. Lead chromate caused sarcoma at the injection site. Injected into muscle, it caused renal-cell carcinoma.

Timing mattered in one striking experiment. Pregnant mice were given lead acetate in drinking water. Dosing ran from gestation day 12 through four weeks after birth. Their offspring developed kidney lesions at rates that rose with the dose. The lesions were atypical hyperplasia, adenoma, and adenocarcinoma.

Not every lead compound did it. Lead naphthenate, lead carbonate, lead arsenate, lead nitrate, and metallic lead powder did not raise tumor rates. IARC found the animal evidence sufficient for inorganic lead compounds as a class. It named lead acetate, lead subacetate, lead chromate, and lead phosphate. It found the evidence inadequate for lead oxide, lead arsenate, tetraethyl lead, and lead powder.

Lead also acts as an accomplice. Rodents given lead compounds developed more kidney tumors, faster, when a second carcinogen was present. Rats fed both lead subacetate and 2-acetylaminofluorene got more kidney and liver cancer than rats fed either one alone.

The mechanism is not settled. Lead does not appear to damage genes directly at the blood levels people actually reach. IARC put it plainly: there is little evidence that lead interacts with DNA at normally encountered blood lead concentrations.

Several indirect routes are proposed. Lead can block DNA synthesis and DNA repair. It can drive oxidative damage through reactive oxygen species. It can bind to proteins. Those include DNA-binding proteins, tumor-suppressor proteins, and the proteins that repair damage. That last one would explain why lead makes other carcinogens worse.

In 1978 the federal government banned the sale of lead-based paint for use in homes. CDC lists chipping or peeling paint in buildings built before 1978 as a source. EPA says lead paint in good shape is usually not a problem. Worn, peeling, chipping, chalking or cracking paint can make chips and dust that are breathed in or swallowed. Doors, window sills, stairways, railings and porches wear the most. EPA advises cleaning up such dust right away with a wet cloth.

Leaded gasoline was the other great source. In the mid-1980s it produced roughly 90% of all lead emissions from human activity. EPA's phase-out began in the early 1970s. By 1988, lead in gasoline was down to 1% of the 1970 level. On-road use was banned outright in 1996. Use in aircraft, racing cars, farm equipment, and marine engines was still allowed.

NTP calls industrial processes, especially lead smelters, now the primary source of lead emissions. It lists jobs with frequent high exposure: battery production and recycling, foundry work, lead smelting and refining, leaded-glass work, and pigment work. House renovators and lead miners have a moderate frequency of high exposure. Construction and demolition workers have a low frequency. The last primary lead smelter in the United States closed in 2013. In 2015, all refined US lead came from recycling, primarily lead-acid batteries.

How much gets in depends on the route. About 30% to 50% of an inhaled dose reaches the bloodstream. Only 8% to 15% of a swallowed dose does. But children absorb more of what they swallow than adults do. Iron, calcium, and zinc in the diet change the figure too. Skin contact adds only a limited amount.

Absorbed lead first spreads to blood plasma, the nervous system, and soft tissue. Then it moves again. Between 75% and 90% of the lead in a body ends up in bones and teeth. There it can sit for decades and leach back out.

CDC says even low levels of lead in a child's blood are linked to developmental delays, learning difficulty and behavior problems. Cutting lead exposure matters whether or not the cancer question is ever settled. If you are worried about a child's exposure, ask a doctor about a blood lead test.

NTP reports OSHA's enforceable permissible exposure limit for metallic lead, inorganic lead compounds and organic lead soaps as 0.050 mg/m³ of air (50 µg/m³). NTP adds a caution: this was OSHA's limit in 2018, but it may not reflect more current studies and may not adequately protect workers. Workers who think they are over-exposed can ask their employer or OSHA about testing and medical monitoring.

Other benchmarks NTP lists sit lower. EPA's air quality standard for lead is 0.15 µg/m³. FDA caps bottled water at 0.005 mg/L. Consumer paint is limited to 90 ppm.

• IARC Monographs Volume 87 — Inorganic and Organic Lead Compounds
• IARC Volume 87, summary and evaluation, PDF
• NTP Report on Carcinogens, 15th edition — Lead and Lead Compounds profile
• CDC — About Lead in the Environment
• EPA — Protect Your Family from Sources of Lead