- Cools the human body
- Transmits stimuli of hearing and smell
- May convey disease agents
- Air polluted by dust, smoke, toxic gases and chemical vapours leads to sickness and death
Human beings need a continuous supply of air to exist. The requirement for air is relatively constant (about l0-20ml per day)
Composition
Air is a mechanical mixture of gases. The normal composition is as follows:
- Nitrogen-78.l per cent
- Oxygen-20.93 per cent
- Carbon dioxide-0.03 per cent
- Argon, neon, krypton, xenon and helium-traces
- water vapour
- Ammonia-traces
- Suspended matter such as dust, bacteria, spores and vegetable debris.
Air becomes impure by
- Respiration of men and animals
- Combustion of coal, gas, oil etc
- Decomposition of organic matter
- Trade, traffic and manufacturing processes which give off dust, fumes, vapours and gases.
The composition of outdoor air is maintained by self cleansing mechanisms operating in nature:
- Wind: Wind dilutes and sweeps away the impurities by its movement and it does not allow them to accumulate in one place
- Sunlight: The atmospheric temperature and sunlight oxidize impurities and kill the bacteria
- Rain: It cleanses the atmosphere by removing the suspended and gaseous impurities
- Plant life: The green plants utilize the carbon dioxide and generate oxygen; the process is reversed during the night time.
When the rate of pollution becomes too high or when the cleansing process becomes ineffective, it constitutes a health hazard.
The Air of Occupied Room
Human occupancy and activity vitiate air in occupied rooms and give a sense of discomfort to the occupants. The changes in air that take place in confined places are both chemical and physical.
- Chemical changes: The respiratory activities of occupants lead to increased carbon dioxide and reduced oxygen. An average person at rest gives off 0.7 c.ft of carbon dioxide per hour, this may increase upto 2 c.ft. during physical activity. In a mixed gathering comprising all age groups, the per capita output of carbon dioxide is around 0.6 c.ft per hour.
- Physical changes: The most important physical changes are
- Rise in temperature: Due to emanation of body heat.
- Increase of humidity: Increased evaporation of moisture from the skin and lungs results in elevated humidity.
- Decrease in air movement: In crowded places, the natural movement of air is impeded.
- Body odours: Unpleasant odours arise from foul breath, perspiration, bad oral hygiene, dirty clothes and other sources. The production of body odours depends upon the social status, age and personal hygiene of the people.
- Bacterial pollution: The exhaled air contains microorganisms in suspension, which are discharged into the air during conversation, coughing, sneezing and loud talking.
Unless the vitiated air is replaced by fresh air, it may adversely affect the comfort, health and efficiency of the occupants. It is known that a feeling of suffocation or discomfort and complaints of headache, drowsiness and inability to concentrate are experienced by the occupants of insufficiently ventilated rooms. There is also the risk of droplet infection and lowered resistance to disease (on prolonged exposure).
Discomfort
Discomfort is a subjective sensation experienced by the people in ill-ventilated and crowded rooms. It was believed to be due to increased carbon dioxide and decreased oxygen, resulting from respiration. However, it is now established that the causes of discomfort are not due to chemical changes but physical changes. Air temperature, humidity, air movement and heat radiation are the physical changes that determine the "cooling power" of the air with respect to the human body. Professor Lee rightly puts it that "The problems of ventilation are physical not chemical, cutaneous not respiratory".
Indices of Thermal Comfort
Thermal comfort is a complex entity. It is expressed in terms of –
- Air temperature: For a long time, air temperature was used as an index of thermal comfort, but it was realized that air temperature alone was not an adequate index of thermal comfort.
- Air temperature and humidity: Later, air temperature and humidity were considered together to express thermal comfort. Even this was found to be unsatisfactory.
- Cooling power: Still later air temperature, humidity and air movement were considered together and expressed as "cooling power" of the air.
- Effective temperature: Effective temperature is an arbitrary index which combines the effect of temperature, humidity and movement of the Internal air on the sensation of warmth or cold felt by the human body.
- Corrected effective temperature: This Index is an improvement over the ‘Effective Temperature Index’. Instead of the dry bulb temperature, the reading of the Globe Thermometer is used to allow for radiant heat.
Comfort zones
Definition: The range of Effective Temperatures over which majority of adults feel comfortable. Comfort zone depends on physical, physiological and psychological factors. Hence, no single temperature can be called as comfort zone. Comfortable thermal conditions are those under which a person can maintain normal balance between production and loss of heat, at normal body temperature and without sweating. Comfort zones evaluated in India are as below:
| Sl. No.
|
Comfort zone
|
Corrected effective temperature (in Centigrade)
|
|
1.
|
Pleasant and cool
|
20
|
|
2.
|
Comfortable and cool
|
20-25
|
|
3.
|
Comfortable
|
25-27
|
|
4.
|
Hot and uncomfortable
|
27-28
|
|
5.
|
Extremely hot
|
28-30
|
|
6.
|
Intolerably hot
|
>30
|
|
Participants