BHOPAL, Times Beach, Agent Orange. Hardly a week passes without a news story about some community disaster traced to human exposure to a hazardous substance, with the latest culprit invariably described as the “the most toxic chemical known to mankind.” In the issue-attention cycle of American politics, toxic chemicals are surely in the ascendancy.  Political entrepreneurs eagerly seize upon the issue of protecting their constituents from chemical assault. Meanwhile, Bhopal is compared to Three Mile Island, and the dump sites scattered in thousands of places around the nation are described as “chemical time bombs.”