How can we know when the air we are breathing is harmful?

In response to public concern, small, relatively inexpensive air pollution sensors are coming on to the market. But tests show that they can be inaccurate. Is there an alternative?If only we could see the air pollution around us we could identify the culprits and avoid exposure. From an early age we are taught not to drink dirty water or eat mouldy food but we have less opportunity to avoid harmful air. In a re-run of autumn 2010, this September’s warm weather caused unusually late summert