Encyclopaedia of occupational health and safety: chemicals, fourth edition. JM Stellman ed. Geneva, Switzerland: International Labour Organization, 1998 Nov; 3:80.3-80.4
Natural rubber (cis-I,4-polyisoprene) is a processed plant product that can be isolated from several hundred species of trees and plants in many areas of the world, including the equatorial regions <;If Africa, Southeast Asia and South America. The milky sap, or latex, of the commercial rubber tree Hevea brasiliensis provides essentially all (more than 99%) of the world's supply of natural rubber. Natural rubber is also produced from Ficus elastica and other African plants in. production areas such as Cote d'lvoire, Madagascar, Senegal and Sierra Leone. Natural trans 1,4-polyisoprene is known as gutta-percha, or balata, and comes from trees in South America and Indonesia. This produces a less pure rubber than the cis isomer. Another potential source of commercial nat4ral rubber production is the guayule shrub, Partheniurn argentaturn, which grows in hot, arid regions, such as the southwestern United States. Production of Hevea rubber is divided between plantations larger than 100 acres and small farms, typically less than 10 acres. The productivity of commercial rubber trees has increased regularly since the 1970s.. This increased productivity is due primarily to the development and replanting of acreage with faster maturing, higher yielding trees. The use of chemical fertilizers and the control of rubber tree diseases have also contributed to the increased productivity. Strict measures for the control of exposures to herbicides and pesticides during storage, mixing and spraying, the use of appropriate protective clothing and barrier creams, and the provision of change rooms and appropriate medical surveillance can effectively control the hazards associated with the use of agricultural chemicals. Rubber trees are usually tapped for latex by making a spiral cut through the bark of the tree on alternate days, although the frequency and method of tapping vary. The latex is collected in cups hung on the tree below the cuts. The contents of the cups are,. tra:TlsfeIitClT6Targe-comamersana'moved"toprocessmgstitions'. ,.- Ammonia is usually added as a preservative. Ammonia disrupts the particles of rubber and produces a two-phase product consisting of 30 to 40% solids. This product is further concentrated to 60% solids, resulting in ammoniated latex concentrate, which contains 1.6% ammonia by weight. A low-ammonia latex concentrate (0.15 to 0.25% ammonia) is also available. The low-ammonia concentrate requires the addition of a secondary preservative to the latex to avoid coagulation and contamination. Secondary preservatives include sodium pentachlorophenate, tetramethylthiuram disulphide, sodium dimethyldithiocarbamate and zinc oxide. The chief hazards to field workers are exposure to the elements, animal and insect bites and hazards related to the sharp tools used to make incisions in the trees. Injuries that result should be treated promptly to reduce the risk of infection. Preventive and therapeutic measures can reduce the hazards of the climate and pests. The incidences of malaria and gastro-enteric diseases have been reduced on modern plantations through prophylaxis, mosquito control and sanitary measures. The guayule shrub, a native plant of southern Texas and north central Mexico, contains natural rubber in its stems and roots. The whole shrub must be harvested for the rubber to be extracted. Guayule rubber is essentially identical to Hevea rubber, except that guayule rubber has less green strength. Guayule rubber is not a viable commercial alternative to Hevea rubber at this time.