Several factors influence toxic effect, one of which is the concentration of a material (Table 8.5). Concentrations may be expressed in terms of percentage or parts per million (ppm) or billion per milligram or kilogram; the higher the concentration, the more serious the effect. (Figure 8.4) illustrates some part-per-million approximations. There are also materials that can have an anesthetic effect. This may range from loss of feeling and sensation to unconsciousness. Anesthetics include nitrous oxide, ethers and hydrocarbons.