Some indoor air contaminants, principally particulates, can be removed effectively by commercially available filtration and removal technology. There are several air cleaners like media filters, electrostatic air cleaners, adsorbers, centrifugal separators, air washers and other absorbers. Most of these devices are relatively expensive and generally suitable for air conditioned buildings only.
The indoor air pollution can be dangerous to health. Good ventilation is the easy option for this problem. Planned layout of kitchen and rooms of public congregation may prevent the pollution. A normal human being gives off 0.3 LPM of CO2 at rest and 1 LPM during physical activity. The exhalations contain about 1 ppm of CO. Concentrations of these pollutants build up in the absence of ventilation to undesirable values.
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Indoor pollution can be controlled by disinfection, ventilation and lighting as follows:
1. Disinfection:
In recent years disinfection of air received much attention. Many of the germicides or disinfectants used can pollute air. Usually the disinfectants used contain formaldehyde which is a highly toxic and irritant gas. It precipitates and destroys protein. It is effective against vegetative bacteria, fungi and many viruses but only slowly effective against bacterial spores and acid-fast bacteria. It does not injure fabrics and metals and may also be used for disinfection of blankets, beds, books and other valuable articles. Triethylene-glycol vapours are found to be effective air bactericides particularly against droplet nuclei and dusts.
Ultraviolet radiation has also been found to be effective in special situations such as operation theatres and infectious disease wards. Since direct exposure to UV rays is dangerous to eyes and skin, the ultraviolet lamps are shaded and are located in the upper portions of the rooms near the inlet of air. UV rays have not proved effective for general use, in public assembly and school rooms. Application of oil to the floors also reduces the bacterial content of the air. Air disinfection is still in the experimental stage.
2. Ventilation:
The concept of ventilation lies not only in the replacement of vitiated air by a supply of fresh out-door air, but also in the control of the quality of incoming air with regard to its temperature, humidity and purity with a view to provide a thermal environment that is comfortable and free from risk of infection.
Among the different types of ventilation, natural ventilation is the simplest system of ventilating small dwellings, schools and offices. In this method the wind direction is made use of (when wind blows through a room it is called perflation and when it is obstructed it is called aspiration).
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This artificial ventilation, used to reduce vitiated air and bacterial density is of four types, namely:
(i) Exhaust ventilation;
(ii) Plenum ventilation;
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(iii) Balanced ventilation; and
(iv) Air conditioning.
(i) Exhaust Ventilation:
In this system air is exhausted to the outside by exhaust fans usually driven by electricity. As air is exhausted, a vacuum is created which induces fresh air to enter the room through windows, doors and other inlets. Exhaust ventilation is generally provided in large halls and auditoria. The exhaust fans are housed in apertures of the external walls, high up near the roofs.
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(ii) Plenum Ventilation:
In this system fresh air is blown into the room by centrifugal fans so as to create positive pressure and displace the vitiated air. Plenum or propulsion system is used for supplying air to air-conditioned buildings and factories.
(iii) Balanced Ventilation:
This is a combination of the exhaust and plenum systems of ventilation. The blowing fan must balance the exhaust fan.
(iv) Natural Ventilation:
A minimum of one or two air changes per hour can often be secured by normal traffic and leakage through walls, floors and ceiling and through or around doors and windows, but previously mentioned energy conservation measures may reduce air infiltration and air change by 50 per cent or more.
Under ordinary circumstances, adequate ventilation can be obtained in residences by natural means with properly designed windows. Openable windows, louvers or doors are needed to ventilate and keep attics, basement rooms, pipe spaces and cellars relatively dry.
The tops of windows should extend as close to the ceiling as possible, with consideration to roof overhang, to permit a greater portion of the room to be exposed to controlled sunlight. The minimum total window or skylight area, measured between stops, for a habitable room should be at least 8 per cent of the floor area and the openable area at least 45 per cent of the window or skylight area. The ventilation of modem buildings is usually dependent on mechanical air conditioning and air recirculation, including controlled fresh air intake.
Some examples and design criteria are discussed below:
a. Schools:
Separate venting of each classroom to the outside is preferred. Good standard specify that the mechanical ventilating system provide a minimum air change of 15 to 20 ft3/min per student to remove carbon monoxide and odours, without drafts. The air movement should not exceed 25 ft/min and the vertical temperature gradient should not vary more than 5°F (3°C) in the space within 5 ft of the floor and 2 ft or more from exterior walls. Temperature should be automatically controlled.
b. Public Areas:
In recreation halls, theaters, churches, meeting rooms and other places of temporary assembly, a system of mechanical or induced ventilation is usually needed to meet the requirement of at least 15 ft3 of clean air per minute per person. Any system of ventilation used should prevent short circuiting, uncomfortable drafts and the buildup of unhealthy levels of air contaminants. Approximately one-third of the recirculated air should be clean outside air.
c. Correctional Institutions:
Where dependence is on natural ventilation, window or other openings should provide an area of at least 12.5 per cent of the floor space of the sleeping, living, educational, and work areas and be located to provide cross-ventilation. Gyms and swimming pools require special temperature, humidity and ventilation controls. If dependence is on mechanical ventilation, 15 to 20 ft3/min per person is recommended. Where air is recirculated, approximately one-third should be fresh, clean outside air.
d. Toilets and Bathrooms:
Bathroom and toilet room ventilation is usually accomplished by means of windows or ventilating ducts. The common specification for natural ventilation is that the window or skylight area be at least 8 or 10 per cent of the floor area and not less than 3 ft3, of which 45 per cent is openable. For gravity exhaust ventilation, vents or ducts at least 72 inch2 in area per water closet or urinal and a minimum of 2 ft3/min of fresh air per square foot of floor area should be provided.
A system of mechanical exhaust ventilation providing at least five air changes per hour of the air volume of the bathroom or toilet room during hours of probable use is usually specified for ventilation where windows, ducts or vents are not relied on or are not available for ventilation. EPA recommends 50 fit3/min per water closet and per urinal for a public restroom. Exhaust fans activated by the opening and closing of doors or by a light switch do not provide satisfactory ventilation. The recirculation of air supplied to toilets, lavatories, toilet rooms, bathrooms and restrooms (also kitchens, laboratories and garages) is generally not permitted.
e. Air Change Measurement:
Air in an enclosed space normally diffuses out and outdoor air filters in at a rate dependent on the tightness of the space or building and wind direction and velocity. The air change can be determined by dividing the volume of air entering an enclosed space or room by the volume of the space or room. For example, if 100 ft3/min enters a room having a volume of 1000 fit3 occupied by five people, there would be six air changes per hour (100 × 60 ÷ 1000) and 20 ft3/min per occupant.
The air change may be measured by use of tracers. Desirable qualities of a tracer gas are detectability, non-reactivity, non-toxicity, neutral buoyancy, relatively low concentration in ambient air, and low cost. The commonly used tracers include nitrous oxide (N2O), carbon dioxide (CO2), helium (He) and sulphur hexafluoride (SF6). Several tracer gas measurement procedures exist, including an American Society for Testing and Materials (ASTM) standard. In the measurement, the tracer is released into the building in a specific manner and the concentration of the tracer within the building is monitored and related to the building’s air change rate. Standardisation of devices is necessary.
f. Monitoring:
The monitoring and measurement of the quality of indoor air can be accomplished by modification, as needed, of equipment used to sample ambient air and occupational exposure and by adapting laboratory equipment and procedures. Passive measuring devices for carbon monoxide, radon, formaldehyde and asbestos, although not accurate, are acceptable.
The Anderson impactor sampler may be used to collect indoor airborne fungi supplemented by plate incubation for colony count and identification. Psychrometers for measuring temperature and humidity and smoke tubes for determining air movement are also generally used. Samplers for volatile organic compounds and continuous samplers are also available.
Standardised methods for the determination of air pollutants in indoor air are listed in Table 11.2:
Air conditioning is defined as ‘the simultaneous control of all or atleast the first three of those factors affecting both the physical and chemical conditions of atmosphere within any structure. These factors include temperature, humidity, air movement, distribution, dust, bacteria, most of which affect in greater or lesser degree human health and comfort’.
In air conditioning the air is first filtered and then saturated with water vapour. The excess of moisture is removed and the air is heated to the desired temperature before it is supplied. The difference between the outside and air-conditioned air temperatures should not be more than 20°F, otherwise it gives a sense of discomfort.
Where there is air conditioning, transition rooms are sometimes provided so that people may not be suddenly exposed to high or low temperatures. The transition rooms help people in getting acclimatised, by stages, to high or low temperatures.
3. Lighting:
Good lighting is essential for efficient vision. If the lighting conditions are not good eyes are put to strain which may lead to fatigue and loss of efficiency. The artificial light should have a colour similar to the day light colour. The source should be steady so that there is no flickering. There should be uniform distribution of light, so that sharp shadows and glare are absent. Ceilings and roofs should have a reflection factor of 80 per cent; walls- 50-60 per cent; furniture- 30-40 per cent and floors- <20 per cent.
By proper planning of towns and buildings the natural light can be efficiently utilised. The general principles that are considered in planning for the best utilisation of day light are orientation, removal of obstruction, location of windows and interior decoration. For example, ceiling should be white, upper portions of the wall should be light tilted and the lower portions to be darker to give comfortable contrast to the eyes.