Large particles, such as sand, dirt and dust are not thought to be a major problem. Indeed unless incredibly large quantities are inhaled, they are not hazardous to health. Small particles, such as those produced by the industrial sector and transportation, are incredibly hazardous to health.
No one really knows how much is safe, however research suggest keeping levels below 10 microgrammes per square meter would provide a low risk to human life (though not completely risk free).
No one can put a realistic limit on how much is acceptable, so all we can do is continue to monitor the levels in all major cities, including the effects. But if we all do our part to help reduce PM it is hoped that the world wide levels will slowly decrease, and with it, the number of reported health issues may start to fall.