PM2.5: Diabetes, Heart Attack, Lung Cancer, Premature Death, etc
Etc indeed. The list of maladies laid at the feet of inhaled particulate matter smaller than 2.5 micrometers (thus PM2.5) is long and growing. You can add diabetes to the list thanks to "Association Between Fine Particulate Matter and Diabetes Prevalence in the United States" and lung function deficits in early childhood too ("Effect of Prenatal Exposure to Fine Particulate Matter on Ventilatory Lung Function of Preschool Children of Non-Smoking Mothers").
Is it a certain type of ultrafine particle that's responsible? Some studies say yes and others say no. In vitro toxicity testing tends to suggest that altered function is due simply to particle size while epidemiology studies tend to cast blame on one sort of particle rather than another though the findings vary from study to study and often conflict (a common problem when looking for weak effect associations). Do the observed effects meet the so-called specificity criterion for causal inference? At first the reported ill effects of exposure were said to be cardiovascular but now everything's in play especially since several studies have linked PM2.5 and Premature Death - All Causes.
So, is PM2.5 a universal toxicant and among the leading causes of death? Or could it be that people who live in urban areas with higher PM2.5 levels tend to have higher rates of unhealthy living? Is there anything good to be said about PM2.5? For example, why do farmers, who are often exposed to high levels of PM2.5, especially from endotoxins (think bits of bacteria), often have lifelong protection from many allergies that afflict those exposed to lower doses?
There's also the question of what's to be done about PM2.5. Farmers produce lots of it what with their gravel roads, grain bins, diesel tractors and plowed fields. EPA intends to regulate PM2.5 down on the farm and much more strictly than in the past but at what cost? And for those who don't like cost-benefit analyses what if the changes needed to reduce farm PM2.5 simply causes generation of ultrafine dust to be shifted elsewhere; and to increased markedly? See "The Environmental Cost of Reducing Agricultural Fine Particulate Matter Emissions".
Finally, of course, there's the issue of whom shall be sued. The finding that a speck of cotton dust from your shirt is as toxic (or as non-toxic, depending on how things shake out) as soot from combusted diesel fuel is an obvious impediment to to the diesel litigation plus there's a new study of truck drivers demonstrating that their presumed PM2.5 mortality may not be due to their work but rather can be, at least in part, explained by ultrafine dust exposures in and around the home: "Long-Term Ambient Multi-Pollutant Exposures and Mortality". Efforts to target other deep pockets will have to wait until science produces more definitive answers about what's to blame and how it can be determined that the PM2.5 in question was the cause in fact of the plaintiff's demise - likely an impossible task since causation in such circumstances is almost certainly the result of a constellation of factors; a constellation to be explored by something called eco-epidemiology. More on that another day.
