The necessity for ventilation of dwellings and other buildings has always existed, though the means adopted and the degree of air change have naturally varied according to the purse, knowledge and ideas of refinement of the inhabitants. Perhaps the more primitive of our forebears needed less change of air in their dwellings, since the greater part of their life was spent outside, but even their mud huts had an opening in the roof to let the smoke out and the air in. Probably the two earliest official pronouncements in the way of ventilation were in the forms of proclamations by James I and Charles I respectively.
Public Control of Ventilation. In recent times local authorities have been given considerable control over the erection of buildings in their districts. The Town and Country Planning Acts enable them to plan towns and control the development of private estates; among the objects in view are the restriction of the density of new houses, the separation of industries from dwellings and the provision of open spaces.
By means of by-laws the local authority can require reasonable air space in front of buildings and behind them; can specify a minimum height for habitable rooms (usually 8 feet); can require a minimum window area for such rooms (a very usual by-law requires this to be at least one-tenth of the floor area of the room, of which half must be made to open). Also, under the Model By-laws, every such room must have either a fireplace opening or a ventilator not less than 30 square inches in area.
<Callout type="important" title="Important">Local by-laws give the minimum that will satisfy the council, not of necessity the ideal conditions.</Callout>
Unfortunately there is no certainty that the windows will ever be opened and that the ventilators will not be stopped up (a not unlikely happening if they are placed in such positions as to cause draughts or are made so large as to cool the air unduly). On these accounts the local authority is quite unable to ensure that there will in fact be an adequate flow of air through rooms.
<Callout type="risk" title="Risk">Local authorities cannot guarantee proper ventilation, which can lead to health issues.</Callout>
There are perhaps few subjects which are in a more unsatisfactory state than that of ventilation, or the removal and dilution of the products of respiration and combustion, though a large amount of experimental work has been done in recent years. Nature helps to purify air through the medium of wind, rain, oxidation and diffusion, and man can do much to check its pollution by insisting on cleanliness and absence of dust in rooms, and on the rooms being well lighted.
Composition of Air. Let us first consider the composition of pure air and the things which subsequently render it impure. The most important constituents are nitrogen, oxygen and carbon dioxide, being roughly four-fifths, one-fifth and 0-04 per cent. respectively. Actually a precise statement of the composition of pure air is not possible, since it is a mechanical mixture of gases, varying according to the locality, latitude, altitude and other conditions, not (as is the case with water) a chemical combination of gases always in one set ratio.
Water vapour is usually present in a natural sample of air in quantities varying (by volume) from 1-2 to 5 per cent., so that it is usually considered best to state the composition of the air, excluding the water vapour. Humphrey (in the Scientific Monthly) gave the following as a fair example of analysis of dry air in a rural district, but in spite of the decimals, they must still be considered as approximate: Percentages by Volume:
- Nitrogen = 78.03
- Oxygen = 20.99
- CO2 = 0.03
- Argon = 0.9328
- Neon = 0.0522
- Krypton = 0.0001
- Xenon = 0.000009
- Ozone = 0.0002
- Hydrogen = 0.01
<Callout type="tip" title="Tip">The composition of air can vary significantly based on location and conditions.</Callout>
Oxygen is an element which combines with nearly all other elements. It is found in the soil, and it is also the principal constituent of water, of which it forms 89 per cent. by weight. It is absolutely necessary to life, its removal from the air, or even a substantial lessening of its proportion, causing death by suffocation. It is necessary for the processes of combustion, and also for the production of artificial light, except electric light. One could not live long in pure oxygen, but doses of it are administered by doctors to patients for short periods in extreme cases.
Nitrogen acts as a diluting influence upon the oxygen. It is itself quite incapable of supporting animal life, but is an essential food for vegetable life. Carbon Dioxide. Carbonic acid gas, or carbon dioxide, though so small in proportion to the quantities of the oxygen and nitrogen in the air, is second in importance only to the oxygen. On its presence depends the existence of all vegetable life. It is a compound of oxygen and carbon, and is usually formed at the expense of the oxygen in the air and of carbon derived from various sources, chiefly animal respiration and the combustion of carbonaceous matter, It is also given off as a product of fermentation and putrefaction, An eminent scientist calculated that volcanoes give off ten times as much carbon dioxide as is derived from all other sources. It is vital to the life of plants, forming their largest food factor. Under the influence of sunlight, they absorb and decompose it, retaining the carbon and giving off the oxygen again, but at night the process stops.
<Callout type="important" title="Important">Carbon dioxide levels can be a good indicator of air quality in enclosed spaces.</Callout>
Standards of Purity. Because, in buildings, CO2 is usually present at the expense of oxygen, it is usually taken as an index to its impurity, a high percentage of CO2 being taken as indicating deficiency in oxygen and increase in the impurities usually accompanying respiration, It is by no means an infallible test, and the air of a mineral-water factory may be very heavily charged with CO2 without any harmful effect on the workers, owing to the fact that there is no appreciable reduction in oxygen content or increase in organic or gaseous impurities. Nevertheless, in the absence of any really convenient index to the condition of the air, it remains a widely used standard of purity.
Medical men consider that, to be efficient, a ventilating scheme should be capable of keeping the percentage of CO2 (if due to respiration) below the limit of 0-06 per cent., and that air, in which the percentage of CO2 is greater than this, is likely to be detrimental to the health if breathed continuously in confined spaces.
Humidity. Water vapour, due to evaporation from the surface of land and sea, is always present to some extent in air, but the amount which the air can keep in suspension depends upon the temperature of the air. If the air is suspending the greatest amount which it can at that temperature the air is said to be “saturated”. Any fall in temperature will then result in the formation of mist, rain or dew, according to the height at which this occurs.
Percentage of Humidity. The moisture content of the air is usually measured as a “percentage of humidity”, the state of saturation representing 100 per cent., while, of course, absolutely dry air would show 0 per cent. humidity. The Hygrometer. The “hygrometer”’, used to measure humidity, is usually a “wet and dry bulb” thermometer, though other forms are also in use.
Too dry an atmosphere inside the building gives rise to discomfort owing to its drying effect on the skin, while a humid atmosphere is apt to accentuate the excess of warmth in an overheated room by preventing the sweat glands from functioning. The evaporation of sweat from the skin normally enables the body to rid itself of excess heat. If, on the other hand, the air is too cold, too much humidity will allow the heat to leave the body too rapidly. This effect is very noticeable out of doors when snow is lying on the ground.
With the thermometer at (say) 31° F., the air is dry and few complain of the cold, but let the temperature rise to 33° F. and the snow begin to thaw, the air soon becomes charged with moisture and far more complaints of the cold will be heard.
Argon, neon, krypton, helium and xenon are comparatively newly-discovered gases. They have no known effect on the animal economy, though new uses are constantly being found for them in science and commerce. As far as ventilation is concerned, these gases may be taken as forming, together with the greater volume of nitrogen, the diluting agent, without which the oxygen would be too strong for human lungs.
Ozone. In the pure air of mountain tops, or near the sea, one finds just a trace of ozone, which is a concentrated form of oxygen produced principally by electric discharges during thunder-storms, the evaporation of sea-water, and the action of certain types of vegetation, particularly fresh and salt water growths. It is also produced artificially by means of electrical discharges, a fact that is sometimes taken advantage of in the practice of ventilation. Ozone possesses both positive and negative properties. Thus it renders the air healthful and invigorating, while on the other hand it acts as a disinfectant, killing harmful germs and acting as a strong deodorant.
Impurities. Hydrogen has been given as a gas generally present in the air. In such tiny proportion as 0-01 per cent. it does no harm, but in larger proportions should be regarded as an impurity, due probably to the decomposition of organic matter or (in houses) to leakage from gas pipes.
The air usually contains small quantities of one or other of the following gases, and in measurable quantities they should be considered impurities: Oxides and acids of nitrogen, due to thunderstorms and plant life. Hydrocarbons, sulphuretted hydrogen, sulphur dioxide, sulphurous and sulphuric acid, chlorine, hydrochloric acid and various ammonia compounds, mainly due to decomposition of organic matter or manufacturing processes, but seldom originating inside the dwelling and of course petrol fumes.
Carbon Monoxide. Carbon monoxide (the miners’ “Firedamp”) is similar to carbon dioxide in its constituents but differing in ratio and in practically every other respect. It is formed by incomplete combustion, emanations from the earth, certain manufacturing processes, and in other ways. Unlike carbon dioxide, it is lighter than air, combustible, and is a dangerous poison of a “cumulative” nature—that is to say, a daily dose so small as to be unnoticed may, if repeated day after day, result in serious illness or death, while 1 per cent. in the air of a room is enough to cause coma and death on a quite short exposure.
Methane. In the neighbourhood of low-lying marshy land, carburetted hydrogen or methane, also known as “marsh gas”, may be found. It is not usually found in or near the dwelling in sufficient quantities to be dangerous.
Dust in the Atmosphere. In addition to gases, minute particles of dust are always present. If one places oneself in a dark room and makes just a pinhole through which a ray of sunlight can pass, one can see these fine particles floating about in the streak of light. Among these matters are particles of fine sand, dried mud, carbon from smoke, iron rust, volcanic dust and other inorganic items, as well as organic impurities, such as fragments of horse litter, hairs, spiders’ webs, spores of fungi, pollen, ete.
It is said that some of the finer particles from an eruption travel round the earth twice before finally settling. Such particles of dust—if sufficiently finely divided and of an innocuous nature—so far from being harmful, help to diffuse the light and warmth of the sun.
Bacteria. There are also always in the air numbers of micro-organisms, known as bacteria or bacilli, or (more popularly) as microbes. These organisms are very minute, it being possible to get a number equal to one hundred times the population of London on 1 square inch. They reproduce very rapidly, it having been stated by eminent bacteriologists that a single microbe gives rise to a progeny equal to about four times the population of London in twenty-four hours. They are not necessarily harmful, but the greater the number in any sample of air, the greater the likelihood of the presence of pathogenic or disease-bearing organisms, For all the processes of agriculture they are essential. Their presence in the soil is necessary for the growth of crops, and without their aid the farmer could not ripen his cheese or make his butter. They are necessary, in fact, in the soil, in the manure heaps, in the barn and in the dairy.
Bacteria are usually found in the air, attached to particles of organic matter, so that the importance of keeping the air of dwellings as free as possible from dust is self-evident.
Effect of Respiration. In the process of respiration, air is inhaled into the lungs, the oxygen being brought into contact with the blood. A large part of the oxygen is converted into carbon dioxide. In the course of twenty-four hours the average man gives off a quantity of carbon dioxide containing as much as a half-pound of carbon. Moisture is also given off to the extent of from about 6 to 27 ounces in twenty-four hours. Organic matter and minute quantities of ammonia are also given off.
Effect of Combustion. In the processes of combustion, among the impurities given off are carbon dioxide and moisture; unconsumed carbon; carbon monoxide, due to imperfect combustion, particularly from coal gas and coke fires; also sulphuric acid and sulphuretted hydrogen from gas which has not been properly purified. It is mainly these last two impurities which are answer- able for spoiling pictures, the bindings of books and wallpapers.
Standards of Air Change. In dealing with ventilation some standard of air change is necessary. It must be remembered that the object of ventilation is not merely to keep the air pure, but to prevent excess of moisture from accumulating and to prevent excess of body heat, as well as to maintain the supply of oxygen and to keep down the harmful bacteria or other impurities. In fact, it is generally found that the physical condition of the air is more important in the judgment of the average person than its chemical purity.
CO2 is usually taken as a standard of impurity, 0-06 per cent. being looked upon as a reasonable maximum allowance, i.e. 0-6 cubic feet in 1000 cubic feet. The air of an average town may be taken as containing 0-4 cubic feet in 1000 cubic feet, and the average adult is said to give off about 0-6 cubic feet of the gas in one hour. At this rate he will, in twenty minutes, raise to the allowable limit of impurity 1000 cubic feet of air, and so should be provided with about 3000 cubic feet of air per hour.
This fact is often put another way, to the effect that he should be given 1000 cubic feet of space and the air should be changed three times an hour. B.S. Code of Practice. This should be looked on as the ideal, rather than a standard, requirement, and the Ministry of Works allowance for dwellings is only 600 cubic feet of air per hour per person for average-sized living and bedrooms, with 2000 cubic feet per hour in the kitchen, if cooking is for up to six persons. The investigations of the Ministry of Works into ventilation are now published as the “British Standard Code of Practice in Ventilation”. It is issued by the British Standards Institution under the aegis of the Ministry of Works.
Students who wish to go further into details of recommended rates of air supply to various types of building and apartment than these pages permit, might well obtain and study the code.
Key Takeaways
- Local authorities have significant control over ventilation standards in buildings.
- Carbon dioxide levels are a key indicator of air quality in enclosed spaces.
- Proper ventilation is crucial for maintaining health and preventing the buildup of harmful gases.
Practical Tips
- Ensure that windows and vents are regularly cleaned to maintain airflow.
- Use carbon monoxide detectors to monitor indoor air quality, especially during winter months when heating systems are in use.
- Regularly check for leaks or drafts around doors and windows to improve overall ventilation.
Warnings & Risks
- Do not rely solely on local by-laws for adequate ventilation; ensure that all openings are functional.
- Be cautious of over-ventilation, which can lead to excessive cooling and increased energy consumption.
- Avoid placing vents or windows in positions where they may cause uncomfortable drafts.
Modern Application
While the historical techniques discussed in this chapter provide valuable insights into maintaining healthy indoor environments, modern technology has improved our ability to monitor and control air quality more precisely. Understanding these principles can still be crucial for survival situations where basic infrastructure is compromised or unavailable.
Frequently Asked Questions
Q: What are the recommended standards for air change in dwellings according to the British Standard Code of Practice?
The British Standard Code of Practice recommends an ideal rate of 600 cubic feet of air per hour per person for average-sized living and bedrooms, with 2000 cubic feet per hour in the kitchen if cooking is for up to six persons. These standards are considered the ideal rather than a strict requirement.
Q: How can I ensure proper ventilation in my home without relying solely on local by-laws?
You should regularly check that all windows and vents are functional and not blocked. Additionally, consider using fans or air purifiers to enhance airflow and monitor indoor air quality with devices like carbon monoxide detectors.
Q: What are the main sources of impurities in indoor air according to this chapter?
The main sources of impurities in indoor air include carbon dioxide from respiration, moisture, organic matter, bacteria, and various gases such as carbon monoxide, sulfur compounds, and methane. These can accumulate in enclosed spaces and affect air quality.