Chuwa,
'Sorry about the links. I tried, and couldn't find them either. I don't know why they don't work currently.
Here's another for NIOSH. NIOSH stands for the National Institute for Occupational Safety and Health, and they do the research on topics of occupational health.
http://www.cdc.gov/niosh/npptl/topics/respirators/
You asked which, particulates or gases, is most hazardous? Well, that depends upon which chemical you select.
CO goes into the bloodstream, and combines with haemoglobin to form a complex which blocks the obsorption of oxygen into the red blood cells. We will naturally get rid of it, but having it there reduces our oxygen carrying capacity--basically our exercise performance on the bike.
Ozone is a really bad actor, which has a very low threshold limit value (TLV) of 0.2 ppm for "Heavy, moderate, or light workloads [<= 2 hours]." For heavy work all day, the TLV is 0.05 ppm. Bicycling would be considered heavy work. It acts on pulmonary function, reducing it by acting on the air sacs of the lungs directly. It also goes into the circulation, and forms "free radicals" which react with other molecules within the body.
Diesel exhaust (and CO for that matter) has been implicated in increased heart attacks. This is the particulate that I'm most concerned about, as it is very small, and gets way down into the air sacs in the lungs.
Other gases can also cause problems. Benzene (TLV 0.5 ppm) and formaldehyde (no 8-hour TLV allowed, ceiling level for 15 minutes of 0.3 ppm) are both carcinogens (leukemia for benzene, and formaldehyde is a suspected carcinogen), and are found in auto exhaust. Nitrogen dioxide has a TLV of 3 ppm, and is both an upper and lower respiratory irritant. Sulfure dioxide had a TLV in 2007 of 2 ppm, with a Ceiling Concentration of 5 ppm, but it is on that year's "Notice of Intended Changes." I looked up the intended change for sulfure dioxide, and it was to discontinue the 8-hour TLV-TWA (Time Weighted Average) and put a Ceiling Concentration of 0.25 ppm into effect for 2008. I have not checked my 2008 TLV booklet to see whether it was incorporated as it is at work. I'll look tomorrow.
The lower the TLV, the worse the chemical is on the body. Here are different websites which documents the chemicals in auto exhaust:
http://www.greenlivingtips.com/artic...chemicals.html
http://www.nutramed.com/environment/carsepa.htm
http://www.epa.gov/region5/air/mobile/auto_emis.htm
Even though there is more breathing resistance with the combined HEPA and organic vapor/acid gas cartridges, I'd still go with them. In highly polluted environments, you will find that the gaseous pollutants will decrease lung function, and you can probably make up for the resistance over time with retaining your vital lung capacity. If you can get the HEPA filters with pleats instead of a single surface (which is what I see on the cycling respirators), it should dramatically cut the breathing resistance through vastly increased surface areas. I see this in my North P-100 filter and prefilter, and I believe it is available with many other respirator manufacturers.
By the way, HAPPY NEW YEAR!!!
John