Tractor ploughs a field in the Coachella Valley, California, dust clouds form.

July 3, 2012

A Caterpillar rubber tracked tractor ploughs a field in Thermal, in the Coachella Valley, California. The dust contributes significantly to the poor air quality in the Valley which is below state and fedral standards. Childhood asthma hospitalization rates in Imperial County are the highest in California. USA © David Bacon/