G. Insecticides and fungicides
Sources and health implications
Pesticides are natural or chemical agents such as insecticides, used to destroy troublesome insects, herbicides for weed control, fungicides to control plant disease, rodenticides and germicides (39). A range of organic chemicals are in use as insecticides and fungicides for timber treatment. They include dieldrin, lindane and benzene hexachloride commonly used as insecticides, and Pentachlorophenol commonly used as a fungicide (17). There are many others in use (31), and some, such as DDT, which have been widely banned. Table 11 shows some of the most widespread chemicals used.
Table 11. Insecticides and fungicides commonly used for timber treatment and their health hazards.
* The TLV of arsenic is 0.01 mg/m3 in ACGIH, 1994 (32)
All of these chemicals are necessarily toxic to the organisms they are intended to combat. If inhaled or ingested in sufficient quantities, they can also be hazardous to the health of those involved in applying them, particularly when used in the form of sprays. Many of them attack the nervous system, or can cause damage to internal organs such as liver and kidney. Some can cause skin reactions. One of them has been shown to be carcinogenic to humans and others are suspected carcinogens (2, 40). It has recently been estimated that as many as 3 million people annually are poisoned by pesticides, of whom perhaps 20,000 die (41). Although most of these casualties arise from agricultural use and only a small proportion derives from the use of pesticides in buildings, the hazard from pesticides’ use in buildings is significant as many of the most toxic pesticides are used for timber treatment, and exposure levels may also be particularly high in the indoor environment. Examples of some toxicity levels for some chemicals used in wood formulations are given in Table 12 (42). LC50 - is the statistically derived exposure concentration of a chemical that can be expected to cause death in SO per cent of a given population of organisms under a defined set of experimental conditions (e.g. a 96 hour fish LC50), and LD50 - is the dose of a toxicant that will kill 50 per cent of a given population of organisms within a designated period of time (42).
Table 12. Toxicity levels for some chemicals used in wood preservation formulations.
* The figures are based on time wrighted averages (TWA)
Factors influencing exposure
As with solvents, the principal risk from insecticides and fungicides is to construction workers. A particular risk is to those involved in the remedial treatment of timber in existing degraded buildings which has to take place quite often in poorly ventilated roof spaces, and using sprays (2). Those involved in the pesticide treatment of buildings to eliminate disease vectors can also be seriously at risk (43).
After application, the rate of emission into the indoor environment is relatively slow, since a characteristic of all pesticides is that they have low volatility. However, if used in conditions of very poor ventilation, the level of exposure to occupants after application can be significant. There is little experimental evidence available on domestic exposures, but it has been estimated that in conditions of poor ventilation exposures approaching occupational exposure limits are possible (17).
Pesticides of various types are also used in timber pre-treatment, posing a potential threat to workers in those enterprises, which in some countries commonly operate under a poor state of control. Because of their persistence in the environment and damage to all forms of life, pesticides must be treated as hazardous waste and disposed of with great care. Many of the most serious and widespread cases of pesticide poisoning occur as a result of spills and casually dumping wastes on uncontrolled sites (41).
Acceptable exposure levels
Specific occupational exposure limits for most of the important pesticides have been proposed by national authorities. Table 11 shows those set for UK by the Health and Safety Executive (44), which are similar to the Threshold Limit Values proposed for the United States of America by ACIGH (see also table 12). But it has been suggested that domestic exposure levels should be set at a level only 1 per cent of the Occupational Exposure Limit to protect vulnerable occupants.
The number of chemicals in use as pesticides and timber preservatives is huge and growing annually, and many of their effects are as yet not clearly identified (45). Because of their known toxicity, an international coalition of groups and individuals who oppose unnecessary use and misuse of pesticides, the Pesticides Action Network, has identified the following thirteen pesticides, some of which are used as wood preservatives requiring strict control (45): Aldicarb, Campheclor (Toxaphene), Chlordane and Heptachlor, Chlordimeform, DBCP, DDT, the “drins” - Aldrin, Endrin and Dieldrin, EDB, HCH and Lindane, Paraquat, Parathion and Methyl Parathion, Pentachlorophenol, and 2, 4, 5-T. All of them are highly poisonous to the nervous system. Since no pesticides are free of potential health hazards if used without proper control, less poisonous wood preservative treatments, those based on synthetic pyrethroids (e.g. permethrin) and inorganic boron compounds, should be used. Protective clothing should be worn when treating or handling treated timber. If treated timber is machined or sanded an efficient dust extraction system should be used and wastes disposed of safely: if dust extraction is not available, dust masks should be used. In remedial treatment of timber, particularly in poorly ventilated enclosures, operators should be provided with respirators (46). Furthermore during treatment the following additional requirements are needed;
• mix or dilute products before use in a well-ventilated area, away from the general public, and label clearly all containers;
There is also a growing school of thought that pesticides are not an efficient long term approach to the preservative treatment of timber, because they penetrate the timber only to a limited extent, and are gradually lost to the atmosphere. Because of this limited life, the contribution claimed by pesticide manufacturers to stemming deforestation by reducing future demand for timber has been challenged (2). Thus elimination of the need for pesticides by design is the recommended alternative. The alternative mitigation strategy is to eliminate by design the condition which pesticides are used to treat. Rotting of timbers can only take place under conditions of high humidity; it can be reduced or eliminated by:
• seasoning timber before use to reduce moisture content below 20 per cent;
Likewise, where pesticides are commonly used for protection against termites, they should be replaced where possible by the use of physical barriers to entry, or by making use of naturally termite-resistant species (48).
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