Several studies have shown that numerous National Priorities List (NPL) sites have been contaminated with arsenic (747), cadmium (791), chloroform (596), or nickel (664). The National Toxicology Program (NTP, 1991) has classified these substances as known human carcinogens (arsenic and certain arsenic compounds) or as substances that may reasonably be anticipated to be carcinogens (cadmium and certain cadmium compounds, chloroform, and nickel and certain nickel compounds). The general population is probably exposed to low levels of these hazardous substances through drinking water, eating food, or inhaling contaminated air. People working or living near industries and facilities that manufacture and use chloroform, nickel, arsenic, or cadmium may be exposed to higher than background levels of these hazardous substances. Multiple pathways of exposure may exist for populations near hazardous waste sites. For example, high levels of chloroform (1,890 ppb) were found in well water near a waste site; high levels of cadmium exposure may exist for individuals living near cadmium-contaminated waste sites.