This table relates relative toxicity to LD50 values. Note that a low LD50 value indicates that a chemical is more toxic than one with a high LD50 value. Other useful information can be obtained from dose-response curves. Many chemicals have a dose-response threshold below which no adverse health effects occur. At higher concentrations, however, these same metals can produce toxic effects.
Some chemicals apparently have no safe threshold levels, as in the case of carcinogens.
Although this would lead to the conclusion that no exposure to carcinogenic materials should occur, in reality, it is impossible to achieve zero exposure. We come in contact with natural carcinogens daily, such as the sun's ultraviolet rays. Therefore, the only realistic course is to keep carcinogenic exposure to an acceptable risk level. Thresholds are useful in determining safe levels of laboratory air contaminants. TLVs have been established for many common laboratory chemicals.