Environment and Health

The term environment implies all the external factors - living and non-living, material and non-material - which surround man. In its modern concept, environment includes not only the water, air and soil that form our environment but also the social and economic conditions under which we live.

For descriptive purpose, environment has been divided into three components, all closely related:

• Physical: Water, air, soil, housing, wastes, radiation, etc. • Biological: Plant and animal life including bacteria, viruses, insects, rodents and animals. • Social: Customs, culture, habits, income, occupation, religion etc.

The key to man's health lies largely in his environment. In fact, much of man's ill-health can be traced to adverse environmental factors such as water pollution, soil pollution, air pollution, poor housing conditions, and presence of animal reservoirs and insect vectors of diseases which pose a constant threat to man's health. Often man is responsible for the pollution of his environment through urbanization, industrialization and other human activities. In 1972 the UN conference on the Human Environment focussed worldwide attention on the environmental hazards that threaten human beings. To facilitate work in this area, WHO has compiled a wide-ranging survey of environmental hazards to human health (1).

The dictionary meaning of the word sanitation is "the science of safe-guarding health." One of the best definitions is that given by the National Sanitation Foundation of the USA, which is as follows: 'Sanitation is a way of life. It is the quality of living that is expressed in the clean home, the clean farm, the clean business, the clean neighbourhood and the clean community. Being a way of life it must come from within the people; it is nourished by knowledge and grows as an obligation and an ideal in human relations'. The term "environmental sanitation" has been defined by WHO as "the control of all those factors in man's physical environment which exercise or may exercise a deleterious effect on his physical development, health and survival".

In the past, sanitation was centred on the sanitary disposal of human excreta. Even now, to many people sanitation still means the construction of latrines. In actual fact, the term sanitation covers the whole field of controlling the environment with a view to prevent disease and promote health. Man has already controlled a number of factors in his environment, e.g., food, water, housing, clothing, sanitation. These controllable factors are those included in the "standard of living". It is the control of these factors that has been responsible for considerable improvement in the health of the people during the past century in the developed countries. However, man's mastery over his environment is not complete. As old problems are being solved, new problems are arising. Air pollution is of growing concern in many urban centres. Industrial growth has given rise to the problem of environmental pollution by industrial wastes. Advances in nuclear technology have produced the problem of radioactive pollution of the environment. The demographic growth and fast urbanization all over the world Therefore, the attainment of a healthy environment is are bringing profound social and environmental changes, becoming more and more complex. The term environmental sanitation is now being replaced by environmental health. Proper environmental health now requires the services of the public health qualified doctor, the epidemiologist, the public health engineer, the town planner, the sociologist, the economist, and the health inspector. A purely medical or engineering approach by itself is no longer sufficient; a combined multi-disciplinary programme of action is needed to achieve a healthy environment.

The purpose of environmental health is to create and maintain ecological conditions that will promote health and thus prevent disease. One of the essential public health care element is safe drinking water and sanitation. In 1990, more than 1 billion people in developing world lacked access to safe drinking water and nearly 2 billion people lacked an adequate system for disposing off their excreta (2). Faeces deposited near homes, contaminated drinking water (sometimes caused by poorly designed or maintained sewerage systems), fish from polluted rivers and coastal waters, and agricultural produce, fertilized with human waste are all health hazards. Water quantity is as important as water quality. Washing hands after defecation and before preparing food is of particular importance in reducing disease transmission, but without abundant water in or near home, hygiene becomes difficult or impossible. The lack of water supply and sanitation is the primary reason why diseases transmitted via faeces are so common in developing countries. The most important of these diseases, diarrhoea and intestinal worm infestations, account for 10 per cent of the total burden of disease in developing countries. In addition, an inadequate water supply increases the risk of schistosomiasis, skin and eye infections, and guineaworm disease. Table 1 shows the percentage of population with access to safe water and adequate sanitation in South-East Asia countries.

Two of the changes needed to achieve "Health for All" are concerned with a healthy environment and healthy lifestyle and require initiatives by the individual, the family and the community. Available data from 138 countries show that safely managed drinking water services (that is, located on premises, available when needed and free from (contamination) was accessible by 74% of the global population in 2020. This translated into 2 billion people still lacking safely managed water services-including 1.2 billion people with only basic services, 282 million with limited services, 367 million using unimproved sources, and 122 million drinking surface water. In the same year, safely managed sanitation services (with excreta safely disposed on-site or treated off-site) were available to only 54% of the global population, leaving 3.6 billion people with insufficient sanitation services. While hand hygiene is one of the first lines of defence against the spread of infectious diseases, 2.3 billion people, or 29% of the global population, lacked a basic handwashing facility with water and soap at home in 2020. For both water and sanitation, safely managed services are enjoyed by a higher proportion of populations in urban areas compared to rural areas (2).

While the available data do not yet allow a global baseline estimate for industrial wastewater generation or treatment, estimates for the generation and treatment of domestic wastewater were available for 128 countries and territories in 2020 (covering 80% of the global population). They show that 44% of household wastewater was discharged without safe treatment (2).

Much of the ill-health in India is due to poor environmental sanitation that is unsafe water, polluted soil, unhygienic disposal of human excreta and refuse, poor housing, insects and rodents. Air pollution is also a growing concern in many cities. The high death rate, infant mortality rate, sickness rate and poor standards of health are in fact largely due to defective environmental sanitation. Improvement of environmental sanitation is therefore crucial for the prevention of disease and promotion of health of individuals and communities. Since more than 65 per cent of the population of India live in rural areas, the problem is one of rural sanitation. The first step in any health programme is the elimination through environmental control of those factors which are harmful to health. The environmental factors which are basic and fundamental to individual and community health are discussed in this chapter.

WATER POLLUTION -

Pure uncontaminated water does not occur in nature. It contains impurities of various kinds - natural and man-made. The natural impurities are not essentially dangerous. These comprise dissolved gases (e.g. nitrogen, carbon dioxide, hydrogen sulphide, etc. which may be picked up during rainfall), and dissolved minerals (e.g., salts of calcium, magnesium, sodium, etc.) which are natural constituents of water following its contact with soil; and suspended impurities (e.g., clay, silt, sand and mud), and microscopic organisms. These impurities are derived from the atmosphere, catchment area and the soil.

A more serious aspect of water pollution is that caused by human activity urbanization and industrialization. The sources of pollution resulting from these are: (a) sewage, which contains decomposable organic matter and pathogenic agents (b) industrial and trade wastes, which contain toxic agents ranging from metal salts to complex synthetic organic chemicals (c) agricultural pollutants, which comprise fertilizers and pesticides, and (d) physical pollutants, viz heat (thermal pollution) and radioactive substances (9).

The indicators of pollution include the amount of total suspended solids, biochemical oxygen demand (BOD) at 20 deg. C, concentration of chlorides, nitrogen and phosphorus and absence of dissolved oxygen.

Even if the source of water supply and its treatment are of a high standard, water pollution may still occur as often happens, due to corrosion of pipe lines, leaky joints and cross connections between water supply pipes and sewage drainage pipes. Surveillance has to be exercised at every point in the distribution system to ensure supply of safe water to the consumer.

Water-related diseases

Man's health may be affected by the ingestion of contaminated water either directly or through food; and by the use of contaminated water for purpose of personal hygiene and recreation. The term water-related diseases includes the classical water-borne diseases. Developing countries carry a heavy burden of water-related diseases, the heaviest being the diarrhoeal diseases. Water-related diseases may be classified as follows:

A. Biological (Water-borne diseases)

1. Those caused by the presence of an infective agent:

(a) Viral : Viral hepatitis A, hepatitis E, poliomyelitis, rotavirus diarrhoea in infants. (b) Bacterial: typhoid and paratyphoid fever, bacillary dysentery, Esch. Coli diarrhoea, cholera. (c) Protozoal: amoebiasis, giardiasis (d) Helminthic: roundworm, threadworm, hydatid disease. (e) Leptospiral: weil's disease

2. Those due to the presence of an aquatic host:

(a) Snail: schistosomiasis (b) Cyclops: guineaworm, fish tape worm. B. Chemical -

Chemical pollutants of diverse nature derived from industrial and agricultural wastes are increasingly finding their way into public water supplies. These pollutants include detergent solvents, cyanides, heavy metals, minerals and organic acids, nitrogenous substances, bleaching agents, dyes, pigments, sulphides, ammonia, toxic and biocidal organic compounds of great variety. Chemical pollutants may affect man's health not only directly, but also indirectly by accumulating in aquatic life (e.g. fish) used as human food. The present concern about chemical pollutants in water relates not so much as to their acute toxic effects on human health as to the possible long-term effects of low level exposure, which are often non-specific and difficult to detect. Further, some of the new pollutants are not easily removed by conventional water treatment or purification processes. In many developed countries where water-borne communicable diseases have virtually disappeared, more attention is now being paid to chemical pollution.

In addition to the above, water is associated with the following:

(a) Dental health: The presence of fluoride at about 1 mg/litre in drinking water is known to protect against dental caries, but high levels of fluoride cause mottling of the dental enamel;

(b) Cyanosis in infant: High nitrate content of water is associated with methaemoglobinaemia. This is a rare occurrence but may occur when surface water from farmland, treated with a fertilizer, gain access to the water supply;

(c) Cardiovascular diseases: Hardness of water appears to have a beneficial effect against cardiovascular diseases;

(d) Some diseases are transmitted because of inadequate use of water like shigellosis, trachoma and conjunctivitis, ascariasis, scabies (10);

(e) Some diseases are related to the disease carrying insects breeding in or near water, like malaria, filaria, arboviruses, onchocerciasis, African trypanosomiasis (10).

While pollution seems to be an inevitable consequence of modern industrial technology, the problem, now, is to determine the level of pollution that permits economic and social development without presenting hazards to health. The evaluation of the health effects of environmental pollutants is currently being carried out as part of the WHO Environmental Health Criteria Programme.

WATER POLLUTION LAW

In India, water pollution is becoming a serious problem. To protect water from being contaminated, the Indian Parliament in 1974 passed the Water (Prevention and Control of Pollution) Act. The Act seeks to provide legal deterrent against the spread of water pollution. The Act is a comprehensive piece of legislation. It provides for the constitution of Central and State Water Boards and Joint Water Boards endowed with wide powers for controlling pollution.

AIR POLLUTION

The phenomenon called "pollution" is an inescapable consequence of the presence of man and his activities. The term "air pollution" signifies the presence in the ambient (surrounding) atmosphere of substances (e.g., gases, mixtures of gases and particulated matter) generated by the activities of man in concentrations that interfere with human health, safety or comfort, or injurious to vegetation and animals and other environmental media resulting in chemicals entering the food chain or being present in drinking-water and thereby constituting additional source of human exposure. The direct effect of air pollutants on plants, animals and soil can influence the structure and function of ecosystems, including self-regulation ability, thereby affecting the quality of life (2). In the past, air pollution meant smoke pollution. Today, air pollution has become more subtle and recognizes no geographical or political boundaries. Air pollution is one of the present-day health problems throughout the world.

Sources of air pollution -

The main sources of air pollution are:

(a) AUTOMOBILES: Motor vehicles are a major source of air pollution throughout the urban areas. They emit hydrocarbons, carbon monoxide, lead, nitrogen oxides and particulate matter. In strong sunlight, certain of these hydrocarbons and oxides of nitrogen may be converted in the atmosphere into "photochemical" pollutants of oxidizing nature. In addition, diesel engines, when misused or badly adjusted are capable of emitting black smoke and malodorous fumes.

(b) INDUSTRIES: Industries emit large amounts of pollutants into the atmosphere. Combustion of fuel to generate heat and power produces smoke, sulphur dioxide, nitrogen oxides and fly ash. Petrochemical industries generate hydrogen fluoride, hydrochloric acid and organic halides. Many industries discharge carbon monoxide, carbon dioxide, ozone, hydrogen sulphide and sulphur dioxide. Industries discharge their wastes from high chimneys at high temperature and high speed.

(c) DOMESTIC SOURCES: Domestic combustion of coal, wood or oil is a major source of smoke, dust, and sulphur dioxide and nitrogen oxides. The London disaster of air pollution in 1952 in which thousands had died was due to domestic coal burning.

(d) The most direct and important source of air pollution affecting the health of many people is tobacco smoke. Even those who do not smoke may inhale the smoke produced by others ("passive smoking").

(e) MISCELLANEOUS: These comprise burning refuse, incinerators, pesticide spraying, natural sources (e.g., wind borne dust, fungi, molds, bacteria) and nuclear energy programmes. All these contribute to air pollution.)

Air pollution monitoring in India -

The National Air Quality 'Monitoring Programme, sponsored by the Central Pollution Control Board (CPCB) since 1990, has generated database over last 14 years in 10 major Indian cities, viz. Ahmedabad, Mumbai, Kolkata, Delhi, Hyderabad, Jaipur, Kanpur, Kochi, Chennai and Nagpur. The programme facilitates evaluation of long-term air quality trends for health-related criteria pollutants such as inhalable dust, sulphur dioxide, nitrogen dioxide, lead, hydrogen sulphide, ammonia and PAH. The trend analysis showed that Suspended Particulate Matter (SPM) exceeds the CPCB standards in all cities most of the time throughout the year. The concentration ratio of < P10 fraction (human respirable particles) to the total SPM varies between 30 to 60 per cent, with coastal cities showing higher percentages. The concentration of respirable suspended particulate matter is invariably higher at the industrial sites. The PAH in suspended particles at various cities did not show specific trend. The BaP concentration was higher than the CPCB standard at Ahmedabad, Mumbai, Kolkata, Delhi and Nagpur in winter months. Chromium, copper, nickel, arsenic, lead, iron, zinc, sulphate, nitrate, chloride, fluoride, ammonia, sodium and potassium are secondary pollutants analyzed for the assessment of dry deposition of air pollutants. Wet deposition of air pollution has also been evaluated by analyzing rain water samples at all monitoring stations. The successive three years data indicates that the first rain event has the maximum concentration of pollutants with low pH values and higher sulphate and nitrate contents. The acid rain phenomenon thus prevails, albeit for a limited period, in the urban atmosphere of Indian cities (9).

Effects of air pollution

About 1.3 billion urban residents worldwide are exposed to air pollution level above recommended limits. Air quality in the developed countries has generally improved in the past two decades, but in many developing countries air quality has deteriorated because of rising industrial activity, increasing power generation and the congestion of streets with poorly maintained motor vehicles that use leaded fuel. Air pollution can affect by two ways:

(a) Health aspects: The health effects of air pollution are both immediate and delayed. The immediate effects are borne by the respiratory system, the resulting state is acute bronchitis. If the air pollution is intense, it may result even in immediate death by suffocation. This has taken place in the air pollution epidemic which occurred in London in 1952. The delayed effects most commonly linked with air pollution are chronic bronchitis, lung cancer, bronchial asthma, emphysema, and respiratory allergies.

Lead poisons many systems in the body and is particularly dangerous to children developing brain and nervous system. Elevated lead levels in children have been associated with impaired neuropsychologic development as measured by loss of IQ, poor school performance and behavioural difficulties (10). Table 3 shows the major air pollutants, their source and adverse effects on health.

Precise estimates of the risk of air pollution to health are difficult to quantify because of problems in estimating the degree of exposure of individuals and the influence of possible confounding variables such as smoking, nutrition. occupation and climate. Air pollution damages the human respiratory and cardio respiratory system in various ways. The elderly, children, smokers and those with chronic respiratory difficulties are most vulnerable. Under the assumption that achievable reduction in urban air pollution can prevent 5 per cent of all infections and chronic respiratory diseases, these reductions could avert 0.6 per cent of the global burden of disease. Epidemiological studies have shown that a sudden increase in the air pollution has often been associated with immediate increase in morbidity and mortality.

(b) Social and economic aspects: These comprise destruction of plant and animal life; corrosion of metals; damage to buildings; cost of cleaning and maintenance and repairs and aesthetic nuisance. Air pollution also reduces visibility in towns. It can soil and damage clothing’s.

Prevention and control of air pollution -

The control of air pollution is ultimately an engineering problem. The WHO has recommended the following procedures for the prevention and control of air pollution:

(a) Containment: That is, prevention of escape of toxic substances into the ambient air. Containment can be achieved by a variety of engineering methods such as enclosure, ventilation and air cleaning. A major contribution in this field is the development of "arresters" for the removal of contaminants.

(b) Replacement: That is, replacing a technological process causing air pollution, by a new process that does not. Increased use of electricity, solar power generation, natural gas, and central heating in place of coal have greatly helped in smoke reduction. There is a move now to reduce lead in petrol which is a cumulative poison. In India also deleaded petrol is being used.

(c) Dilution: Dilution is valid so long as it is within the self-cleaning capacity of the environment. For example, some air pollutants are readily removed by vegetation. The establishment of "green belts" between industrial and residential areas is an attempt at dilution. The capacity for dilution is, however, limited and trouble occurs when the atmosphere is overburdened with pollutants.

(d) Legislation: Air pollution is controlled in many countries by suitable legislation, e.g., Clean Air Acts. Legislation covers such matters as height of chimneys, powers to local authorities to carry out investigations, research and education concerning air pollution, creation of smokeless zones and enforcement of standard for ambient air quality. To decrease the nuisance of air pollution, the Government of India have enacted "The Air (Prevention and Control of Pollution) Act" in 1981.

(e) International action: To deal with air pollution on a world-wide scale, the WHO has established an international network of laboratories for the monitoring and study of air pollution. The network consists of two international centres at London and Washington, three centres at Moscow, Nagpur and Tokyo and 20 laboratories in various parts of the world (13). These centres will issue warnings of air pollution where and when necessary.

Disinfection of air

In recent years, disinfection of air has received much attention. The methods employed are:

(1) MECHANICAL VENTILATION: This reduces vitiated air and bacterial density.

(2) ULTRAVIOLET RADIATION: This has been found to be effective in special situations such as operation theatres and infectious disease wards. Since direct exposure to ultraviolet rays is a danger to the eyes and skin, the ultraviolet lamps are shaded and located in the upper portion of the rooms near the inlet of air. Ultraviolet rays have proved effective for general use in public assembly and school rooms.

(3) CHEMICAL MISTS: Triethylene glucol vapours have been found to be effective air bactericides, particularly against droplet nuclei and dust.

(4) DUST CONTROL: Application of oil to floors of hospital wards reduces the bacterial content of the air. Air disinfection is still in the experimental stage.

NOISE -

Noise is often defined as "unwanted sound", but this definition is subjective because of the fact that one man's sound may be another man's noise. Perhaps a better definition of noise is: "wrong sound, in the wrong place, at the wrong time". Man is living in an increasingly noisy environment. The 20th Century has been described as the "Century of Noise". Noise has become a very important "stress factor" in the environment of man. The term "Noise Pollution" has been recently coined to signify the vast cacophony of sounds that are being produced in the modern life, leading to health hazards.

Sources: The sources of noise are many and varied. These are automobiles, factories, industries, air-crafts etc. Noise levels are particularly acute near railway junctions, traffic round-abouts, bus terminuses and airports. Use of pressure horns, recreational noise of loudspeakers with full volume during festivities particularly at night are other sources of noise production. The domestic noises from the radios, transistors, T.V sets - all add to the quantum of noise in daily life.

Control of Noise

A variety of approaches may be needed to control noise. These include:

(1) CAREFUL PLANNING OF CITIES: In planning cities, the following measures should be taken to reduce noise;

(a) Division of the city into zones with separation of areas concerned with industry and transport; (b) The separation of residential areas from the main streets by means of wide green belts. House fronts should lie not less than 15 metres from the road and the intervening space should be thickly planted with trees and bushes; (c) Widening of main streets to reduce the level of noise penetration into dwellings.

(2) CONTROL OF VEHICLES: Heavy vehicles should not be routed into narrow streets. Vehicular traffic on residential streets should be reduced. Indiscriminate blowing of the horn and use of pressure horn should be prohibited.

(3) TO IMPROVE ACOUSTIC INSULATION OF BUILDING: From the acoustic standpoint, the best arrangement is construction of detached buildings rather than a single large building or one that is continuous. Installations that produce noise or disturb the occupants within dwellings should be prohibited. Buildings should be sound-proof where necessary.

(4) INDUSTRIES AND RAILWAYS: (Control of noise at source is possible in industries. Special areas must be earmarked, outside residential areas, for industries, for railways, marshalling yards and similar installations. When these demands cannot be met, protective green belts must be laid down between the installations and residential areas.

( (5) PROTECTION OF EXPOSED PERSONS: ( Hearing protection is recommended for all workers who are consistently exposed to noise louder than 85 decibels in the frequency bands above 150 Hz. Workers must be regularly rotated from noisy areas to comparatively quiet posts in factories. Periodical audiogram check-ups and use of ear plugs, ear muffs are also essential as the situation demands.

( (6) LEGISLATION: Many states have adopted legislation providing for controls which are applicable to a wide variety of sources (4). Workers have the right to claim compensation if they have suffered a loss of ability to understand speech.

( (7) EDUCATION: (No noise abatement programme can succeed without people's participation. Therefore, their education through all available media is needed to highlight the importance of noise as a community hazard.

RADIATION

1) Natural sources: Man is exposed to natural radiation from time immemorial. Natural background radiation arises from three sources:

(a) Cosmic rays: The cosmic rays which originate in outer space are weakened as they pass through the atmosphere. At ordinary living altitudes, their impact is about 35 mrad a year. At altitudes above 20 km cosmic radiation becomes important. It has been calculated that a commercial jet pilot receives about 300 mrad per year from cosmic radiation (1).

(b) Environmental:

(i) Terrestrial radiation: Radioactive elements such as thorium, uranium, radium and an isotope of potassium (K40) are present in man's environment, e.g., soil, rocks, buildings. It is estimated that man derives about 50 mrad per year from terrestrial radiation. Areas exist (e.g., Kerala in India) where there are rock formations containing uranium, it can be as high as 2,000 mrad a year,

(ii) Atmospheric radiation: The external radiation dose from the radioactive gases radon and thoron in the atmosphere is rather small about 2 mrad per year. (c) Internal radiation Man is also subjected to internal radiation, i.e., from radioactive matter stored in the body tissues. These radioactive materials include minute quantities of uranium, thorium, and related substances, and isotopes of potassium (K40), strontium (Sr), and carbon (C). Internal radiation is thought to inflict about 25 mrad a year on the body as a whole, but may be as high as 70 or 80. All in all, it is estimated that the total natural radiation to which the average person is subjected comes to approximately 0.1 rad a year.

(2) Man-made sources: In addition to natural radiation, man is exposed to artificial or man-made sources. These are:

(a) X-rays: The greatest man-made source of radiation exposure to the general population at the present time is medical and dental X-rays. Two distinct groups are involved: (i) patients and (ii) radiologists and medical technicians. When optimum radiographic techniques are employed, the skin dose to the patient from a single X-ray film varies roughly from 0.02 to 3.0 rad.

(b) Radioactive fallout : Nuclear explosions release a tremendous amount of energy in the form of heat, light, ionizing radiation and many radioactive substances, the important being the isotones of carbon (C14), iodine (1131), cesium (CS137) and strontium (Stop). CS137 and Stop are considered most important because they are liberated in large amounts and remain radioactive for many years. The "half-life" of Sro is about 28 years and that of CS137 is 30 years. These radioactive particles released into the atmosphere float down to earth for some years afterwards. Because of air currents, the particles are distributed fairly evenly over the whole human race. Measurements made in 1963 in Germany (F.R.), a country where there had been no explosions, showed that a dose of 33 mrems per person was received from this source.

(c) Miscellaneous: Some everyday appliances (e.g., TV sets, luminous wrist watches) are radioactive. But radiation from these sources at present is too small to be important.

Radiation protection -

The amount of radiation received from outer space and background radiation has been estimated to be 0.1 rad a year. Apparently, this does not at present constitute a hazard. The additional permissible dose from man-made sources should not exceed 5 rad a year. Of the man-made sources, the X-rays constitute the greatest hazard. In routine fluoroscopy, a dose of 4 rad is delivered to a part of the body in about one minute. This implies that unnecessary X-ray examinations should be avoided, especially in the case of children and pregnant women (5). It also implies adequate control and surveillance of X-ray installations, protection of workers, improvement in techniques and improvements leading to dose reduction (6). Effective protective mea… and is called radiation hygiene. The International Commission on Radiological Protection (ICRP), the International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) have been active in this field The ICRP's recommendations on radiation dose levels (7) for occupational workers and for the general population have been adopted by many countries. It has been recommended that the genetic dose to the whole population from all sources additional to the natural background radiation, should not exceed 5 rems over a period of 30 years. The WHO has published permissible radiation levels in drinking water. The IAEA has sponsored many symposia on radioactive waste disposal and associated subjects (8). Its main concern has been to promote peaceful uses of atomic energy and to assure that these uses do not imperil peace or health. In short, there has been a worldwide interest in preparing safety standards, codes of practice for the safe operation of nuclear power plants and enunciating the basic principles of radiation protection.

SOLID WASTES

The term "solid wastes" includes garbage (food wastes) rubbish (paper, plastics, wood, metal, throw-away containers, glass), demolition products (bricks, masonry, pipes), sewage treatment residue (sludge and solids from the coarse screening of domestic sewage), dead animals, manure and other discarded material. Strictly speaking it should not contain nightsoil. In India and similar other countries, it is not uncommon to find nightsoil in collection of refuse (3). The output of daily waste depends upon the dietary habits, life styles, living standards and the degree of urbanization and industrialization. The per capita daily solid waste produced ranges between 0.25 to 2.5 kg in different countries.

Solid waste, if allowed to accumulate, is a health hazard because:

a) it decomposes and favours fly breeding b) it attracts rodents and vermin c) The pathogens which may be present in the solid waste may be conveyed back to man's food through flies and dust. d) there is a possibility of water and soil pollution, and e) Heaps of refuse present an unsightly appearance and nuisance from bad odours.

There is a correlation between improper disposal of solid wastes and incidence of vector-borne diseases. Therefore, in all civilized countries, there is an efficient system for its periodic collection, removal and final disposal without risk to health.

Arthropod-borne diseases

Arthropod-borne diseases constitute a major health problem in India. Malaria continues to be an important vector-borne disease with an annual morbidity of 4 to 5 million cases. Filaria is another important arthropod-borne disease with an estimated 236 million people living in filaria-endemic areas. Scabies is a widespread disease, especially in rural areas. Dengue, haemorrhagic fever, Japanese encephalitis and KFD are also among the important arthropod-borne virus diseases in India. The prevalence rates for trachoma which is a major cause of blindness in India vary from 0.5 per cent in West Bengal to 79 per cent in Punjab and Haryana. Thus arthropods are responsible for much ill-health and deaths .

Transmission of arthropod-borne diseases

Three types of transmission cycles are involved in the spread of arthropod-borne disease

(1) DIRECT CONTACT: In this method of spread, the arthropods are directly transferred from man to man through close contact, e.g., scabies and pediculosis.

(2) MECHANICAL TRANSMISSION: The disease agent is transmitted mechanically by the arthropod. The transmission of diarrhoea, dysentery, typhoid, food poisoning and trachoma by the housefly are examples of mechanical transmission of the disease agent by the vector.

(3) BIOLOGICAL TRANSMISSION: When the disease agent multiplies or undergoes some developmental change with or without multiplication in the arthropod host, it is called biological transmission.

This may be of three types:

(a) Propagative: When the disease agent undergoes no cyclical change, but multiplies in the body of the vector, transmission is said to be propagative, e.g., plague bacilli in rat fleas.

(b) Cyclo-propagative: The disease agent undergoes cyclical change, and multiplies in the body of the arthropod, e.g., malaria parasite in anopheline mosquito.

(c) Cyclo-developmental: When the disease agent undergoes cyclical change but does not multiply in the body of the arthropod, e.g., filarial parasite in culex mosquito and guineaworm embryo in cyclops.

In communicable disease terminology, the word vector means an "arthropod or other invertebrate which transmits infection by inoculation into or through the skin or mucous membrane by biting, or by deposit of infective materials on the skin or on food or other objects". The period of time necessary for the development of the disease agent in the arthropod host is called extrinsic incubation period. For example, the extrinsic incubation periods in malaria and filaria are from 10 to 14 days or longer depending upon the environmental temperature. The host in which the sexual cycle of the agent occurs is called the definitive host, e.g. mosquito is the definitive host in malaria. The host in which the asexual cycle of the agent occurs is called the intermediate host e.g., mosquito in filaria and cyclops in guinea-worm disease. By infestation is meant the lodgement, development and reproduction of arthropods on the surface of the body or in the clothing e.g., louse infestation.

Principles of arthropod control-

The general principles of arthropod control are:

1. Environmental control 2. Chemical control 3. Biological control 4. Genetic control

(1) Environmental control: This offers the best approach to the control of arthropods, because the results are likely to be permanent. Examples of environmental manipulation are: elimination of breeding places (source reduction); filling and drainage operation; carefully planned water management; provision of piped water supply; proper disposal of refuse and other wastes; cleanliness in and around houses, etc. Intensive health education of the public as well as political support are essential prerequisites.

(2) Chemical control: A wide range of insecticides belonging to the organochlorine, organo-phosphorus and carbamate groups of compounds (Fig. 17, page 909) are available for vector control. It must be mentioned that vector control by insecticides alone is no longer fully effective because resistance has appeared in over 100 species of arthropods of public health importance. This coupled with the danger of environmental contamination has led to restricted use of many insecticides in some countries. To avoid undue environmental pollution, it is now considered essential to replace gradually the highly persistent compounds such as DDT with compounds which are readily "biodegradable" and less toxic to man and animals such as methoxychlor, abate and dursban (1). As there is no alternate control method which is as efficient and economical as the insecticides, it is postulated that most of the developing countries will have to depend, for sometime to come, on the organochlorine compounds for the control of vectors.

(3) Biological control: To minimize environmental pollution with toxic chemicals, great emphasis is now being placed on biological control. The use of larvivorous fish especially Gambusia is well known in mosquito control. Fungi of the genus Coelomomyces are also known to be pathogenic to mosquitoes. A variety of other biological agents (e.g., bacteria, fungi, nematodes, protozoa and viruses) are under study for the control of insects. But the fear exists that the introduction of biological agents for the control of arthropods may pose a direct hazard to the health of man himself (2).

(4) Genetic control: Much progress has taken place in recent years in the theoretical and applied aspects of genetic control of arthropods. The WHO/ICMR Research Unit at New Delhi has contributed massively to the techniques of genetic control of mosquitoes (3). Techniques and chromosomal translocations have been found to be such as sterile male technique, cytoplasmic incompatibility effective in small field trials. In conclusion, it may be stated that these methods are nowhere near the stage where they can be used large-scale in an effective way.

(5) Newer methods: sought for pest control. These are -

(a) Insect growth New and innovative methods are being regulators (b) Chemosterilants (c) Sex attractants or pheromones (4).

Integrated approach-

Since no single method of control is likely to provide a solution in all situations, the present trend is to adopt an "integrated approach" for vector control combining two or more methods with a view to obtain maximum results with the minimum effort and to avoid the excessive use of any one method.