Purity Of Air
Life Support
In order to maintain life, the human body must be kept at a steady temperature of 37°C. Careful measurements on astronauts have shown that the normal internal base temperature varies by less than 0.3°C among different people.
Since the human body does not register temperature, wind velocity and humidity readings, per se, the question arises how the body registers and regulates comfort. The answer to this question is not known. In fact, we do not even know where human internal temperature sensors are located nor how they function.
All five senses are active in the indoor habitat and have to be satisfied within a narrow range between deprivation and over stimulation or pain. Judgement of indoor air quality is, as we have indicated, highly subjective and related to many factors. At the dinner table, for example, food odours are as important as taste. In aeroplanes, however, food orders are generally perceived as inappropriate.
Temperature and Touch
Temperature is usually associated with touch, although tactile contact is not necessary for the perception of radiative heat. Touch receptors are distributed over the entire body, but their density varies. The sense of touch is poorly developed in many people and operates on a very muted level in schoolchildren, who are notoriously insensitive to heat the cold.
On a cold day, for example, the radiation from a fireplace is perceived as comfortable, but on a hot day the same stimulus causes discomfort. Likewise, on a cold day the consumption of ice water or ice cream causes the blood to flow in such a way as to counteract the chill to the internal organs, whereas on a hot day it sends a stop signal to the skin glands responsible for regulatory sweating. Conversely, when hot tea or coffee is consumed, the internal sensors initiate sweating even on a cold day. This response is not paradoxical, since the function of the temperature sense is to maintain an adequate gradient, or to adjust the gradient, or heat flux, rather than to maintain a constant skin temperature.
Very little is yet understood about this amazing sense, which must keep the temperature of the core of the body stable within + 0.5°C. Cases of partial deficiency, common with the other senses, are not observed, because any error of this sense would induce fatal changes in metabolism.
Sight
The eye is the most thoroughly schooled sense. Thus, visual perception often overrides that of other senses. For example, we tend to equate sunshine with warmth, even if we are exposed to a chilly wind or air conditioner. Although the physical nature of light is well known and light can be accurately analysed, we still know relatively little about the way the eye and brain translate frequencies into colours.
Hearing
A healthy young ear can recognise sound vibrations over a frequency range from about 16 to 40,000Hz, a range of 12 octaves. In addition, the ear can voluntarily discriminate and de-emphasise background noise, such as the gushing or the blood in the skull, the hum of an air conditioner or ventilator in a concert hall, or traffic noises. Further more, it can analyse a sound into individual musical instruments or voices in a group. Yet on an oscilloscope a sound signal appears as a simple bleep. It is not known how the brain deciphers it, but again psychologists claim that people function best if they experience a wide range of sound stimuli.
Many features in buildings are designed to reduce noise and echo effects. If properly chosen and placed some, such as carpets and drapes, can enhance indoor air quality by serving as buffers for humidity; other merely collect dust and create suspended particulates.
Smell and Taste
Odour and taste are chemical senses, activated by contact between osmogenic molecules and geminal nerves.
The human nose is extremely sensitive and can discriminate among literally hundreds of different molecules; it can sharply distinguish between certain chemical functional groups such as acids, aldehydes, esters and ketones, as well as among the various substituents of benzene and other aromatic compounds. Smell also differs from the other senses in that it is strongly time dependent: steady odours become less obvious. Thus, people in hospital rooms rapidly become indifferent to odours that are so strong they repel visitors. This characteristic, which enables people to live with their own internal and external body odours, deprives them of a defence against toxic gases that accumulate slowly.
Summary
Although still very poorly understood, the human senses constitute a vital link between our environment, our comfort and our behaviour. One reason for this is that our senses serve life-support needs that cannot be expressed in simple physical parameters. They respond to differences, concentrations and gradients of quantities which change periodically. Furthermore, an individual’s perception of comfort changes as a function of body functions. Thus, stimuli that are perceived as pleasant at one time can be almost intolerable at others. Consider, for example, the gourmet who relishes raw onions at dinner, but finds their odour repulsive at the breakfast table; the executive who needs bright lights at work, but favours a dimly lit room in which to read a novel at home; or teenagers who thrive on loud music but are prevented from sleeping by the hum of a fan or air conditioner.
Obviously, human beings require variety and an evenly lit office with a steady noise level and constant temperature and humidity would be stultifying. Yet the environmental engineer, architect and ventilation engineer have to find a range of stimuli appropriate for all employees, whether they perform physical or mental work and whether they smoke or not.
Problems in homes can also involve formaldehyde, microbes, radioactive radon and pesticides, but the major indoor problems are moisture, combustion gases, especially carbon monoxide, nitric oxide and smoke from tobacco smoke, kitchen stoves, heating ranges and wood stoves and fires.
Monitoring of Air Quality
Normally, building occupants have to rely on their five senses and such signs as odour, dry throat and burning eyes to detect indoor air pollution. Unfortunately, our senses are better at judging comfort than at judging the threshold of toxicity. They are reliable for only a few molecules, such as ozone, nitric oxide or formaldehyde, whereas the more dangerous carbon monoxide, particulates, asbestos, and sulphates are undetectable far beyond safe limits.
Air Quality Control
Most indoor air pollution problems can be lessened or solved by increased air mixing by ventilation, by eliminating indoor sources and adjusting our activities, or by cleaning recirculated air. There is no sure or easy way to find the best mix. The foremost consideration in all buildings is to provide adequate transfer of oxygen and metabolic products. In residential buildings, the main problem is humidity control. A family of four releases about 201 of water from perspiration, respiration and household activities per day. This volume requires 1300m3 of air at 75°F/23°C and 50% relative humidity (RH).
In reality, however, the water is produced during the two short mealtime and bathing peaks; instead of vanishing slowly, it migrates to cold spots where it condenses and remains stubbornly hidden in the form of moisture in the building materials, while most of the indoor air rapidly becomes normal or dry.
Condensation is a very serious problem if buildings designed for freely flowing air are suddenly retrofitted with insulation or vapour barriers to reduce heating costs. This condensation threatens the health of both the basic building structure and the occupants. The only adequate solution is to ’seal’ the building and to provide intentional forced or natural ventilation at a rate adequate to mix air fully and remove excess moisture. Furthermore, buildings that rely on mechanical ventilation should not rely on uncontrolled infiltration, but should provide either natural cross-draught ventilation, or forced air circulation or both. Further, it is vital that such buildings have an appropriately placed air intake through which air may be admitted, either continually or in batches, as desired. In any case, natural ventilation should always be provided.
Basic Needs
In order to stay alive, a sedentary person has to inhale about 10,000 breaths of air each day. This adds up to about 10-20m3/day of air and about 0.7kg of oxygen, which is used to fuel metabolism. The air should be fresh and clean because the human respiratory system is a very sensitive and efficient transmitter of gases, benign or poisonous and of fine dust.
This air is exhaled at 37°C and 100% humidity. We further need to dissipate waste heat at a rate of 80W in order that the heat content of the body remains constant and its core temperature be maintained at 37 + 0.3°C. This stability is achieved partly by perspiration, which contributes about 2 litres of water to indoor air. The exchange depends on activity.
Furthermore, each day we need to dissipate about 0.5kg of carbon dioxide and traces of some 200 chemicals; the latter are responsible for normal body odours. A lightly clothed sedentary person feels most comfortable at 24.5°C, 40% humidity and an air flow of about 0.25m/sec, but individual comfort differs considerably, depending on activity level. If the temperature is not suitable, the body can adjust its level of heat dissipation by such defence mechanisms as increased metabolic activity and sweating or shivering. Comfort also depends on cultural expectations. The following table shows the temperature proposed by the war restoration board in England in 1945. In 1931, 60% of residences had unheated outdoor privies to keep odours away from the house. Today, a pleasant year-round temperature of about 24°C is expected in all residences.
Obviously, life-styles in industrialised nations are changing rapidly. The average life expectancy in the United Kingdom has increased from 38 years in 1850 to 47 years in 1900 and 75 years today.
British Standard of Warmth in Homes 1945
Room Temperature (°C) ————- Humidity (%)
Living Room 16.20 —————— 30-65
Bedroom 12.14———————- 30-65
Kitchen 15.5 ———————— 70
Hallways 7.2-10 ——————— 30-65
Bathroom 13-14
From Vent Axia -Ventilation Hand Book