It has been said that atmospheric particle loading have decreased
severl-fold during the last 40 years but the health impacts of
particles have not changed significantly during the same period and
may, in fact be greater. How can this be so?

I don't know the answer to your question, but here's a good,
informative link to have a look at:

http://www.epa.gov/airtrends/pm.html

Some things to consider:

--exposures that did damage to people years ago are just showing up
now as health problems, so that even though particulate levels are
down, health problems may not be (yet).

--overall particulates may have decreased, but certain types of
especially problemattic particles may have stayed steady or even
increased

--populations could have been sensitized to particulate pollution, so
that even lower levels can be causing problems

Just some food for thought....good luck with your assignment.