This is Steve Ember with the VOA Special English Agriculture Report.

Farmers have long put animal waste on their fields. They may also use human waste. Modern ways to process（处理）waste and make it safer to use have only been developed since the nineteen-seventies.

Treated（已经过处理的）waste products are called biosolids, or sludge. They contain nitrogen（氮）, phosphorus（磷）and potassium（钾）. Biosolids are removed from the wastewater systems of cities and other communities. They are treated in several ways to kill organisms that could spread disease.

In the United States, the government says sixty-percent of all treated solid waste is used to fertilize land. This includes some farmland. But the government says only one-tenth of one-percent of American farmland uses this kind of fertilizer from year to year.

There are two kinds of biosolids. Class A is considered free of any organism that could be a danger to health. These organisms include viruses, bacteria and worms. Class B biosolids are not completely free of such organisms. So their use is restricted.

Another concern about biosolids is that they may contain chemical pollution. Many kinds of chemicals can enter wastewater systems. In the United States, the Environmental Protection Agency has rules about the presence of nine chemicals in biosolids. The E-P-A requires testing for levels of substances like arsenic（砷）, chromium（铬）, mercury（水银）and lead.

After five years of study, the E-P-A recently decided not to require producers and users of biosolids to add dioxins（二氧化物）to this list. Dioxins are a group of organic chemicals that can stay for a long time in the environment. There are concerns about greater risk of cancer and other disorders.

Dioxins are released by burning plastics and fuels like coal, oil and wood. They are also released by some kinds of chemical manufacturing. Paper production with the use of chlorine（氯）produces dioxins. So does cigarette smoke.

Environmental groups and others condemned the decision not to set rules for dioxins in biosolids. They point out that the use of sludge products is a leading cause of dioxin in the environment in the United States.

The E-P-A agrees that dioxins are highly poisonous. But the agency has also found that levels in the environment have been greatly reduced in the last thirty years. It says there is not enough risk of new cancer cases in farmers and other people to support new rules.

This VOA Special English Agriculture Report was written by Mario Ritter. This is Steve Ember.