Because they're spoiled!
Parts per million sound small but consider this The Environmental Working Group has put it into perspective for all of us. Small amounts DO make an impact. We are part of the food chain , the food web or whatever term you choose to use. What we eat , breath and touch effects us. I live in the Gulf region and am angered each time I see that there are those who would want us to believe it is o.k now. It clearly is not.
This video on toxins should inspire us to try to live more simply and give our children a chance at living a healthy, God given life.