In 1978, a team of scientists from Dow Chemical found that rats exposed to very low levels of dioxin developed liver cancer. This research, combined with earlier tests that demonstrated birth defects in mice at extremely low exposure levels, led to dioxin's characterization as "the most toxic synthetic chemical known to man". Unlike pathogens in sewage sludge which are supposed to be reduced by treatment methods, dioxin in sewage sludge is not reduced by those treatment technologies. The result is dioxin in the soil continues to build up with each additional application of sludge to agricultural lands.