J. Earthen and traditional materials
Sources and health implications
House design and the choice of building materials have a strong influence on the spread of a wide range of infectious human diseases. Although it is the vectors which they can harbour rather than the materials themselves which are responsible for the diseases, in many cases selection and treatment of the materials are central to the programme of control (53).
The environment in and around dwellings provides an attractive habitat for a wide range of arthropods in that they provide shelter from climatic extremes, shade, stability, and an abundant source of food. A number of these arthropods are vectors of human diseases’ pathogens. They include houseflies and cockroaches, triatomine bugs, and domestic ticks, bedbugs and house dust mites. Some colonise humans and animals directly, while others breed outside the house but enter it to feed (54).
The most important disease carried by vectors is perhaps the American form of Trypanosomiasis, or Chagas’ disease, which is transmitted by the bites of the triatomine bug. There are around 13 to 15 million people in Latin America infected by this debilitating disease with about 100 million at risk (30). The disease is caused by a parasite Trypanosoma Cruzi, that can be carried in the bugs’ faeces. The faeces are deposited where the bug feeds, and the parasite can then get into the victim’s blood stream through the bite injury. The parasite lives and reproduces inside the human body, particularly the heart. People inflicted with the Chagas’ disease are often unable to work because of the damage to their cardiovascular system. Health effects of other arthropods include: plague and typhus (fleas); shigellosis, salmonellosis, and viruses - hepatitis A and poliomyelitis (cockroaches); relapsing fever (soft ticks); viral hepatitis B (bed bugs); and house dust allergy (dust mites) (54). IARC has evaluated hepatitis B virus (chronic infection with) as carcinogenic to humans, group 1 (28).
Factors influencing exposure
Research has indicated that the type of building materials used has an important influence on the spread of diseases. In a rural housing study carried out in Venezuela, for example, 200 traditional houses of mud and wattle were compared with the same number of newer houses made of concrete blocks. It was found that while 55 per cent of the traditional houses revealed the presence of triatomine bugs, they were present in only 9 per cent of the newer houses (55). Generally, where the dwelling is made from low-strength masonry in the form of unstabilised earth blocks, rammed earth or stone in earth mortar, or of mud and wattle, the walls are very prone to cracking as the earth dries, providing suitable dark spaces for disease vectors to hide. Plastered walls are less prone, as long as the plaster is maintained uncracked. Soil floors can also be a source of suitable cracks. Roofs made from palm thatch are also a problem as thatch provides plentiful hiding spaces. In Venezuela it has also been found that the eggs of the Chagas’ disease vector are often stuck to the palm fronds used for thatching. Traditional flat roofs of poles piled with brushwood and covered in a thick layer of mud are used in upland areas of Argentina and Bolivia where nights are cool. These also have been found to provide an ideal habitat for triatomine bugs (54). Furthermore, the diseases associated with these arthropods are particularly prevalent in tropical areas, since higher temperatures enable the disease vectors to breed more rapidly.
Methods available to eliminate infestation include spraying the walls and roofs with insecticides (however it should be noted that use of insecticides to eliminate infestation gives problems associated with insecticides); plastering walls with smooth materials; and replacing wall and roof materials with smooth crack free materials. Spraying campaigns have had some success, but where the surface of the wall is absorbent, as is often the case with unstabilised mud walls, the absorbency can reduce the surface amount of the active ingredient to which the insects are exposed to the point at which it is ineffective (56). Spraying also needs to be repeated frequently to be effective. This option may not be feasible to the poor population of the developing countries since the prices of insecticides are beyond their reach. An alternative and effective method of eliminating infestation is through the application of a smooth, durable plaster layer. One study from Brazil reports complete elimination of triatomine bugs largely due to the use of kaolin clay to produce strong smooth walls resistant to cracking (57). But the choice of materials for plastering which are compatible with earthen base materials is difficult. Cement-based plasters rarely adhere to mud walls because of the differential moisture movement.
Replacement of traditional earth and thatch materials with denser, more stable materials is often advocated as the best means to eliminate pest infestation. The least cost alternatives to earth or stone based walls most widely available are either fired brick or concrete block laid in cement mortar. Thatch roofs can be replaced by corrugated galvanised iron sheets. These materials are being very widely adopted in any case, particularly in urban areas. But using them is by itself no guarantee of protection against pest infestation, unless the building is well-built. And selection of these materials has considerable implications beyond disease control: it is much more difficult to maintain comfortable living conditions without using ceilings (which may, if used, negate all the benefits by providing new pest habitats); the cost of these materials is often prohibitive, leading to smaller built space and consequently overcrowding; and cement and fired clay manufacture are heavy users of commercial energy contributing to urban and atmospheric pollution (58).
Alternative lower cost and lower-energy materials are becoming available which could provide a solution - stabilised soil for floors and walling materials, and fibre concrete tiles for roofing, making use of local vegetable fibres. Extensive trials of these materials have been conducted in different countries in recent years, and low-cost equipment is now available to enable them to be produced at low cost in small scale operations and with minimal use of commercial energy or factory-made additives (59, 60, 61). Caution should, however, be made that substitute materials could have their own health risks. For example the substitution of traditional roofing and walling materials to cement based and clay fired products still exposes those involved in their production to harmful effects of dusts and gases. In Tanzania, for example, all the 15 dust samples which were collected in three factories (a ceramic factory, a cement factory, and a kaolin quarry) indicated that the quartz content exceeded the acceptable threshold limit value of 0.1 mg/m3 thus suggesting that the exposed workers had a high risk of developing silicosis (62).
[Ukrainian] [English] [Russian]