Post 433 — by Gautam Shah
Comfort of users in an interior environment depends on:
1. Temperature of Environment and our Body
2. Level of Humidity,
3. Air velocity,
4. Quality of Air.
● TEMPERATURE OF ENVIRONMENT AND OUR BODY need to reach some equilibrium. In an Interior space, environmental temperature is modulated by the built form and its amenities. Direct solar radiation heats up various objects, depending on their thermal capacity, conductivity, colour, texture, etc. Heated objects radiate this heat, as long wave radiation, back into space after the main source has ceased its input. Delayed heat releases create very complex patterns of heat gain. A temperature profile in any space is rarely even and this causes different gradients and convective air movements. Such air movements affect the rate of evaporation and the level of humidity in a space, rate of ventilation, and quality of air.
Temperature is the major determinant for comfort level. Air temperature determines the rate at which our body will exchange heat with the environment. Rate of heat exchange affects the metabolic activity of the body and as a result its work capacity, fatigue and recovery schedules. In a temperature range that is acutely different from the acclimatized one, our body has to work harder to adopt to the situation.
● LEVEL OF HUMIDITY is the amount of vapour held by air at a particular temperature. With a rise in temperature, the expanded air accommodates more humidity, whereas a drop in temperature condenses the humidity. Proportion of humidity at any given temperature affects the rate of evaporation and the heat exchange of the body through perspiration. Excessive loss of humidity from body, and high proportion of humidity in air, both affect our sense of well being and comfort.
In hot and humid climate high level of humidity does not allow adequate heat dissipation through evaporation of the perspiration. As a result body temperature increases and it has to resort to other methods of heat dissipation. In hot arid climates the low level of humidity causes rapid evaporation. Body cannot cope up with such rapid loss of moisture, as it has limited amounts of water available within it. In cold arid climates the body has no excessive amounts of heat requiring dissipation through high perspiration, however, the low level of humidity removes even the moisture that helps the skin to remain soft and supple. In cold humid climates even minute perspiration does not evaporate readily, and in excessively cold climates it may cause a frost bite.
Air with high percentage of humidity is comparatively deficient in oxygen and may cause problems to people with TB or asthma. Low level of humidity removes the moisture from the nostril, reducing its filtering capacity to keep out the airborne pollutants.
● AIR VELOCITY is caused by temperatures and pressures differences across locations, within connected spaces. Air can also become mobile due to fans or such air circulating devices. Air velocities go below 0.15 m/s, and most people complain of stuffiness, in spite of all other parameters of comfort being satisfactory. Air velocities above 1.5 m/s, are annoying, such as papers being blown out or dust stirred up. People may tolerate such extreme (high and low) air velocities under very hot-humid and cold-dry conditions or for the sake of body thermal management.
Draughts are low velocity air movements, which occur due to temperature and air pressure thresholds near cracks and such leakage points in enclosed spaces. In enclosed spaces, draughts are not always perceptible, as they do not cause any sensation of pressure on skin. Draughts, however, help in convective heat exchange, evaporation and dilution of pollutants in air. Draughts cause localized cooling or heating of sensitive organs of our body. Such sensation on feet is a common experience in trains, buses, sofas, undersides of office tables, etc. Children and aged people with deficient blood circulation and body temperature regulatory mechanisms, are readily affected by such currents.
Breeze is a low to medium velocity air movement, which affect only local areas. The breeze does not let air borne particulate matter to settle down. Breeze causes effective pressure on skin, with immediate and very perceptible skin sensation, which can be avoided by appropriate screening and deflection.
Winds are very high velocity movements of air. Winds affect large regions, and few interior spaces. Winds raise particulate matter in the air, cause rapid change in level of humidity and cause discomfort due to high pressure sensation on the skin. In hot and cold both types of climates people often close all the openings to reduce the air movements and thereby control the convective heat gain or loss.
Turbulent air velocity is less comfortable than a laminar air velocity. Turbulent air movement achieves a better mix of air, whereas laminar helps in greater displacement of air mass. This is the reason why in hot arid climates small size opening is used to create turbulence or a viscous flow, and in hot humid climates body level openings help a laminar flow to displace the humidity.
● QUALITY OF AIR represents the health supporting conditions of interior spaces. Pollution of internal air occurs due to occupation of space by people, plants and pets (exhalation, body odours, excretion product odours, food preparations), gadgets and equipments, building and furnishing materials. The quality of air is usually determined by people’s sensation to various odours present in the air. But certain harmful pollutants like carbon dioxide and radon cannot be perceived by people at high concentrations.
Quality of air is determined in two ways. There are absolute standards that provide for ideal conditions for comfort and bio survival. Relative standards provide ways for determining the qualitative difference between out door air and indoor air. Quality of outside air is generally superior because an infinite space and high speed winds are available for dilution to occur. Quality of internal air can be improved by diluting the proportion of pollutants in air, by replacing part of the fouled air with comparatively cleaner air, or by various mechanical and chemical scrubbers.
Particulate matter is a major source of air pollution, which mainly but not necessarily, originates outside and penetrates inside through various cracks and openings. Particulate can be dust, fumes, mist or biogenic matter. Particles of diameters greater than about 75 microns settle out rapidly and are termed grit. Particles of smaller than 50 microns may remain suspended and constitute aerosols. An aerosol is a liquid or solid particle which is in a quasi stable suspension in air. Very fine aerosols may remain suspended for weeks, whereas larger aerosols may get deposited in minutes. The deposition of very small particles (2 microns) is influenced by temperature gradient (through convective currents). The effect on health due to airborne particulate matter of biogenic origin such as fungi, moulds, bacteria, viruses, pollens are well known.
In modern artificially controlled environment buildings are well designed and sealed to eliminate waste leakages. Such spaces function well so far as environmental systems operate. Pollutants arising from building materials, aerosols settling down, degradation of biotic materials, evaporation of condensed moisture from air handling plants, etc. continue to be added to the internal environment but at night time, on off days and when there are power breakdowns, there is no casual ventilation.