For those interested in the technical side of this issue, here is some additional information.

The research involves Particulate Matter 2.5 (PM2.5).  Particulate Matter is the solids or liquids emitted into the air. The 2.5 represents 2.5 microns in diameter or less.  A micron (or micrometer) is a millionth of a meter.

Here is an interesting article that discusses the science behind the new rule:

SCIENTIFIC OBJECTIONS TO PROPOSED REGULATIONS FOR IN-USE ON-ROAD DIESEL VEHICLES
Dr. Matthew Malkan. Professor of Physics and Astronomy, UCLA,  Dec. 10, 2008

CARB, like any powerful autonomous government agency, needs to follow this guiding principle:

The more costly the proposed regulations, the higher the degree of scientific certainty required to justify them.

CARB’s proposed new regulations on diesel exhaust go far beyond what any of the other 49 states, or the federal government has adopted.  The claimed toxic effects of diesel particulate matter (roughly described as “PM2.5”) are hundreds of times smaller than, for example, the increased risk of lung cancer caused by cigarette smoking.  These possible effects are so small, the actual exposure levels of human subjects are so difficult to estimate, and there are so many confounding health factors that are impossible to control, that the entire question needs to be broadly re-assessed before adopting a radical crash program of harsh new regulations on diesel trucks.

I’ve tried to take an objective look at the scientific question: Is fine particulate matter in diesel exhaust causing cancer and premature deaths of a measurable number of Californians?  The short answer is that we do not yet know.  But whichever way it eventually turns out will have no effect whatever on my career, or my grant funding.  I’m just a 30-year L.A. resident looking for a clear answer, but I do use the statistical tools of epidemiologists (e.g., Cox Proportional Hazard tests) in my own astrophysics research.

The APPENDIX details the case that CARB’s scientific evidence is too flimsy to justify its proposed regulations.  In summary, CARB’s advisors’ original justification for targeting PM2.5 was that it could cause lung cancers.  This claim has not been confirmed by subsequent research.  In fact the research that CARB relies on has failed to find any statistically significant increased risk of ANY form of cancer, or other lung diseases associated with fine particles.  In a classic “bait and switch”, CARB then sought new correlations with the far larger, amorphous category of deaths due to heart disease, without a clear medical model of how this might be caused by fine particles.

Even in the studies CARB advisors chose to weight most heavily (the ones they tend to be co-authors on), the claimed associations between PM2.5 and “premature” deaths of almost any kind range from insignificant to barely “significant” at the 95% confidence level. In the physical sciences, you can’t get a result published unless it passes the higher 99% significance level. I doubt any of the studies of mortality risks of fine particles has that statistical confidence.  This stricter requirement is particularly necessary because–as the reviewers repeatedly admit–the true uncertainties in their results are almost always underestimated, so that the significance of their findings is overestimated.