Particulates in the air -- how much is too much?

If the most polluted city in the world has an average particulate matter measurement of 169 micrograms per cubic meter (Cairo, Egypt), we are in bad shape here in Temecula. We have a reading of 180 today and some of the nearby cities are even as high as 220 or more. I wrote to the City of Temecula to ask what standard is used to determine whether people should stay inside -- is there one? What standard is used to close schools or cancel outdoor activities? I know the air is bad -- I can feel the burning in my throat even though I've been inside all day. I wonder what the long term effects of extended exposure to this are going to be? I wrote more about the measurements on my other blog. I'd like more information but I don't know where to get it.