The WHO has guidelines which will be of interest. They give the rationale to PM2.5 and PM10 exposure limits. These limits are measured as average hour and 24 hour means respectively.
Like noise, particulates is all about the dose. The higher the dose, the higher the risk of health consequences of the exposure. Shorter times means one can be exposed to a higher concentration of particulates (like noise) before damage is caused. Long term exposure the limits are less as there is a compounding effect of time.
