Hi JR8 -
I'm just learning about this myself, having gone out mask-less on Saturday and Sunday....
Here's something from the US EPA: http://www.epa.gov/pmdesignations/faq.htm#0
Taken from this website:
"Particles less than 10 micrometers in diameter (PM10) pose a health concern because they can be inhaled into and accumulate in the respiratory system. Particles less than 2.5 micrometers in diameter (PM2.5) are referred to as "fine" particles and are believed to pose the greatest health risks. Because of their small size (approximately 1/30th the average width of a human hair), fine particles can lodge deeply into the lungs.
One group at high risk is active children because they often spend a lot of time playing outdoors and their bodies are still developing. In addition, oftentimes the elderly population are at risk. People of all ages who are active outdoors are at increased risk because, during physical activity, PM2.5 penetrates deeper into the parts of the lungs that are more vulnerable to injury."
And from other things I've read in the last 24 hours:
If you have asthma or similar pulmonary issues, or if you have heart disease, a high PM2.5 reading can be especially problematic in either/both the long and/or short run, as not only do these particles lodge in the lungs, but they are also small enough to cross over into the bloodstream.
I was also watching ChannelNews Asia this morning, and they had an MD on who said that it was perhaps more important to pay attention to the PM2.5 readings than the PSI (although that is also important). This may be especially important if you fall into any of the above risk categories.
I'm by no means well versed in this, but hope this info helps -