Spld cyclist

Well, yes, at the instant you're within a few feet of a bus exhaust, your exposure is greater than when you are 50 feet from a bus exhaust. However, your whole commute won't be within two feet of a bus exhaust. Your actual exposure will be from breathing an average of higher and lower concentrations for varying lengths of time. Depending on the route you take and the time you take it, the average concentration you breathe on your commute may be higher or lower than what is measured at a fixed monitoring location. I can't speak to how all the sites for monitors were chosen, but the purpose of them is to evaluate compliance with national ambient air quality standards. Therefore, US EPA doesn't want the states to intentionally place them so that air quality problems are missed.

All this doesn't mean that looking at monitoring data is pointless. It's actually very useful, and it's orders of magnitude better than the situation even ten years ago. Now someone with asthma, heart disease, and lung disease can use the internet and have a reasonably accurate basis for deciding when they should exercise and when they shouldn't, whether they should use respiratory protection, or whether it would be better to stay home versus running errands, etc. It isn't perfect, but it is very valuable.

jolly_ross

Agree. This is why I want a particulate filter that can dangle around my neck on a lanyard, for me to stuff in my mouth when I get caught behind a bus, then spit it out when far enough away. This way the filter doesn't have to be able to provide full-effort breathing capacity, as it is I tend to hold my breath behind an idling bus - with this I could kind of half hold my breath.