EPA human testing and the toxicity of PM2.5

I've just added this site to the sidebar. The video introduces the lawsuit:



As the video says, The Environmental Protection Agency is being sued for exposing human subjects to levels that greatly exceed its own daily exposure limits in a number of experiments, in which the subjects are exposed to   – and not just any subjects but those who are particularly at risk. This has come to light since the hospitalisation of one of the subjects led to a FOI request that exposed data concerning many others. Those participating in the tests were not notified that the EPA considered that PM2.5 was something for which (just like secondary smoke) there is 'no safe level of exposure'. 'Particulate matter causes premature death. It doesn’t make you sick. It’s directly causal to you dying sooner than you should', says Lisa Jackson of the EPA (see video).

This whole scenario puts the EPA on a sticky wicket. They are lying about the PM2.5 being toxic, or they are lying to test participants by not revealing the full scale of the risks involve in taking part in the tests. They are also expected to explain the necessity for the tests, since PM2.5 is already regulated.

The condemnation of PM2.5 ('no safe level') is based on particle size, but it seems to stretch credibility to state that just because something is microscopic, it is necessarily more dangerous than something with a larger particles – particle size is only one factor in assessing toxicity. (In the EPA experiments, it is alleged that diesel exhaust was used.) I am not a chemist and cannot assess whether it is an important factor. I can understand finding small particles a particular danger as they are more easily absorbed into the body, but not how the EPA can exclude a reckoning of actual toxicity when reaching the conclusion that there is no safe level of something. It is indeed tempting to conclude that the EPA, in declaring that there is no safe level of  exposure to PM2.5, is searching for excuses to regulate. (This discussion covers this assumption over about the first twenty minutes.)

My question is, if there is no safe level of PM2.5, how is there any justification of outdoor smoking bans, if all the background particulates are also so dangerous?

I look forward to more developments on this.