How Much Asbestos Is Too Much?

Lots of fibers, for years and years.

No asbestos was found in the air after Wednesday’s steam pipe explosion by New York’s Grand Central Station, though debris and dust in the area did contain the carcinogenic mineral. City officials said that brief exposures are “very unlikely” to cause long-term health problems. Just how much asbestos does it take to make you sick?

Usually, it takes years of continued exposure to high levels of asbestos—like those in an industrial environment—to cause health problems. People working under those conditions are more likely to develop lung cancer, mesothelioma, asbestosis, or abnormalities in the lining of their lungs. (According to an EPA review of asbestos data, factory workers exposed for a year to a significant dose of 44 asbestos fibers per cubic centimeter of air had their rates of lung cancer go up by 2.8 percent.) Scientists have plenty of data on these severe cases but much less information about what happens when you inhale small amounts of asbestos—e.g., at rates of less than one fiber per cc of air. At low levels of exposure, the effects may depend on the type of asbestos fiber inhaled, as well as the genetic makeup of the victim and whether he or she is a smoker.

But anecdotal evidence shows that very low levels of exposure can make you sick decades down the road. In Libby, Mont., a town whose largest employer for 70 years was a vermiculite mine, the CDC found rates of asbestosis in the population that were 40 to 60 times higher than expected. Since these illnesses take so long to crop up in general, it’s hard to gauge original levels of exposure. Researchers believe that in some cases, the victims had exposure below what current OSHA standards allow in the workplace.

According to federal rules, employers must make sure there’s less than 0.1 asbestos fiber per cubic centimeter of air averaged over an eight-hour workday. Workers can be subjected to levels of one fiber per cubic centimeter over periods of half an hour. (In industries like plastics manufacturing, which involves asbestos, the limits are pushed up to half a fiber per cc over the course of a workday, or 2.5 fibers for half an hour.) For any commuters caught in Wednesday’s blast, though, the health risk was essentially zero.

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Explainer thanks Vikas Kapil of the Centers for Disease Control and Prevention, Aubrey Miller of the U.S. Environmental Protection Agency, and William Rom of NYU Medical Center and Bellevue Hospital.