Judging the Safety of Nanosilver

Last week brought a decision by the 9th Circuit Court of Appeals that EPA violated their own rules when judging the significance of health risk to toddlers from exposure to a nanosilver antimicrobial product applied to fabric. The Natural Resources Defense Council sued EPA over their conditional registration of a fabric treatment containing nanosilver, arguing that EPA wrongly concluded the product does not pose a significant health risk to consumers.

According to the Woodrow Wilson Center’s Consumer Products Inventory, there are 390 products on the market today containing, or claiming to contain, nanosilver. What are these products? Toothpaste, cookware, air purifiers, socks, coated door knobs. The larger issue is not whether the registered product met requirements for a conditional registration, it is whether to use nanosilver on these types of products. There are both benefits (preventing disease transmission) and risks (anti-microbial resistance, potential effects on the environment).

The judges upheld much of EPA’s determination, but on the issue of the conclusion decided:

The issue here, that the Margin of Exposure (MOE) met, but did not exceed, a safe exposure level, is not a technical one. It is what Fred Klaessig of Pennsylvania Biosystems would call “regulatory chemistry” or something close to it. The judges ruled EPA violated their own guidance -  because when they rounded from a risk level of 0.000497 to 0.0005, the MOE went from 1006 to 1000. Then the MOE was at, not above, 1000 – the issue is one of interpretation of the level. To explain, there is no difference in health risk between a MOE of 1000 or 1006. Either way, the estimated exposure levels are 1000 times below a level of concern. But, because of rounding  - from two significant figures to one – the courts ruled EPA wrongly concluded the risk threshold was met. The decision states, “Although EPA’s point is well taken as a practical matter, it is irrelevant as a legal matter.” EPA made a judgment call that is consistent with the spirit, if not the letter, of their guidance.

It is important that major decisions about approving nanoscale materials (and chemicals in general) are carefully scrutinized to ensure workers, consumers and the environment are not harmed or at risk of harm from products containing them. The court’s decision to vacate EPA’s determination to grant the conditional registration is not based on a finding that nanosilver poses a health risk. It is based on the layering of uncertainty factors leading to a rounding error that dips the margin to the threshold, and now sends EPA back to reach a different decision.  These seemingly small technical determinations underly a much larger societal discussion about antimicrobials in the environment, one that with this ruling, is certain to continue.