Inorganic Lead and Cancer

Lead is element 82. It is a soft silver-blue metal that does not rust away and does not break down. Whatever lead enters the environment stays available to be breathed or swallowed.

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 carbonate, lead chromate, lead oxide, and about a dozen more do not.

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 went back to 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.

The federal government banned consumer uses of lead-based paint in 1978. Some states acted earlier. Homes built before 1978 are more likely to contain it, usually buried under newer coats. EPA says intact lead paint is usually not a problem. Paint that is peeling, chipping, chalking, cracking, or damp is a hazard needing immediate attention. Windows, sills, doors, stairs, railings, and porches shed the most dust.

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.

Industrial sources now lead the list, especially smelters. The jobs with the highest potential exposure are mining and smelting. Also lead-acid battery production, pigment production, construction, and demolition. The last primary lead smelter in the United States closed in 2013. Nearly all refined U.S. lead now comes from recycling, mostly of car 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 contributes little.

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.

That is why lead's best-established harm is not cancer at all. It is damage to the brain and nervous system, especially in children. Cutting lead exposure pays off whether or not the cancer question is ever settled.

OSHA's lead standard is 29 CFR 1910.1025. It defines "lead" as metallic lead, all inorganic lead compounds, and organic lead soaps, and excludes other organic lead compounds.

• Permissible exposure limit: 50 µg/m³ of air, averaged over 8 hours. For longer shifts the limit drops to 400 divided by the hours worked.
• Action level: 30 µg/m³ over 8 hours, measured as if no respirator were worn.
• Biological monitoring: blood lead and zinc protoporphyrin at least every 6 months. Every 2 months once a result reaches 40 µg per 100 g of whole blood, until two consecutive results fall below 40.
• Medical removal: required at 60 µg per 100 g of whole blood, confirmed by a follow-up test within two weeks. Also required when the average of the last three tests reaches 50, unless the most recent result is under 40.

The National Toxicology Program attaches a caution to figures like these. OSHA's limits were accurate as enforceable numbers in 2018. But they may not reflect more current studies, and may not adequately protect workers. Other benchmarks 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
• OSHA 29 CFR 1910.1025 — Lead
• EPA — Protect Your Family from Sources of Lead