Yes, I'm familiar with exactly how small an atom is. Identifying a single lead atom, and measuring harm caused by ingesting a single lead atom, are beyond the

Yes, I'm familiar with exactly how small an atom is. Identifying a single lead atom, and measuring harm caused by ingesting a single lead atom, are beyond the capability of our tools at the moment. You can form a hypothesis that one atom would cause no harm, I can form one otherwise, but until we're able to quantify that then these are just hypotheses. Among lead levels we are able to detect and measure the quantity of, there's no known safe level. And there very well may be points below which the harm caused, is so small that it's some people consider it an acceptable risk, as with your airplane and radiation example.