Even though I live in a heavily populated county of 3.5 million, the traffic along most of the roads that I use is fairly light and there are few diesel trucks. The county transit system has replaced most of the diesel buses with CNG buses, which spew less than half the pollutants and particulates of diesels. Even the garbage trucks and street sweeper vehicles (my favorite city workers) have CNG vehicles. The biggest breathing hazard here is the general air pollution levels. Most of the time the AQI is between 50 and 100. This time of year, AQI gets above 100 fairly often when the wind slows down. Luckily this year we have not had any large wildfires in the nearby hills. Last year we had massive firestorms that dumped incredible particulate pollution into the air, resulting in a rainfall of ashes for almost 2 weeks. I was off the bike for that time last year, because the particle levels were so bad they exceeded the maximum amount of the air quality index scale. I had to wear particle breathing masks to walk to the bus stop and from bus stop into work.