Indoor air quality Definition

There is no strict definition of indoor air quality (IAQ) and no instruments measure it. ANSI/ASHRAE Standard 62-1989 states: ‘For comfort, indoor air quality may be said to be acceptable if not more than 50% of the occupants can detect any odour and not more than 20% experience discomfort, and not more than 10% suffer from mucosal irritation, and not more than 5% experience annoyance, for less than 2% of the time.’

Pollutants in occupied rooms

These come from six sources:

(1) The occupants, who emit carbon dioxide, water vapour, solid particles (lint from clothing, flakes of skin, hair), odours and biological aerosols.

(2) Smoking, which produces carbon monoxide, carbon dioxide, gases and vapours, solid particles, liquid droplets, volatile organic compounds (VOCs) and odours. Many of the emissions are short-term and long-term risks to health.

(3) Building materials, furnishings and their emissions (structural components, surface finishes, furniture, adhesives used in furniture, upholstery, carpets, dusts, house mites).

(4) The activities of people and the equipment they use (printers, copiers, fax machines, computers) give off gaseous and solid emissions and produce odours. The surface coatings of equipment emit pollutants and all electrical equipment may become hot, burn its coating of dust and emit odours. VOCs may also be produced.

(5) Air supplied from outside for ventilation may be contaminated. Hence outdoor air quality should be assessed if necessary. The direct measurement of the concentration of carbon monoxide in the outside air is the best index because CO is well correlated with the presence of oxides of nitrogen, aromatic hydrocarbons and other urban pollutants.

(6) The air handled by the plant and supplied to the occupied space through the ducting may be contaminated. Dirty air filters, particularly when wet, and silencers are potential sources of fungi. Air cooler coils and their condensate trays can provide bacteria, viruses and fungi. If self-draining and properly and regularly cleaned they are safe. Humidifiers using spray water or wetted surfaces are a worse problem because of the continuous presence of liquid water. They must never be used, but dry steam is safe for humidification. The ducts may harbour dust and give contamination if not properly cleaned. Other possible pollutants are: toxicity, radioactivity, materials with infectious or allergenic potentials, the presence of irritants, extreme thermal conditions and objectionable odours.

Effects of humidity

Berglund (1998) observes that humidity affects the evaporation of water from the mucosal and sweating bodily surfaces, influencing its diffusion through the skin. Low humidities, with dew points less than 2°C, tend to give a dry nose and throat, and eye irritation. A dusty environment can exacerbate low humidity skin conditions. Liviana et al. (1988) found eye irritation increased with the passage of time in dew points less than 2°C. High humidities support the growth of pathogenic and allergenic organisms, certain fungi, mycotoxins and house mites. Their growth is enhanced by the presence of high cellulose materials such as fibreboard, dust, lint, skin particles and dandruff (even when these have a low nitrogen content). Fungal contamination is likely when the humidity exceeds 70 per cent.

Ventilation effectiveness (Ev)

This is defined as the fraction of the air introduced from outside that reaches the occupied zone. Some systems of air distribution are poorly designed and their effectiveness can be as low as 0.5. Since the occupants sometimes breathe only 1 per cent of the outside air introduced, poor ventilation effectiveness is bad. The CIBSE (1999) suggest values of ventilation effectiveness for different air distribution arrangements.


ASHRAE (1997) state that, for tobacco smoke, an increase in humidity at constant temperature lowers the intensity level of an odour but an increase in temperature at constant moisture content lowers the odour level only slightly. Initial adaptation to an odour is fairly rapid and perception decreases with time. On the other hand irritation of the eyes and nose generally increases as time passes and is greatest at low humidities. To minimise eye irritation humidity should be from 45 per cent to 60 per cent at normal temperatures. The minimum perception of odours generally appears to be in the humidity band 45 per cent to 65 per cent, at normal temperatures.

Perceived indoor air quality

Cooler, drier air is perceived as being freer of contaminants, even in clean, non-odourous, well-ventilated spaces, and perceived air freshness decreases with increasing temperature and humidity according to Berglund and Cain (1989). Fang et al. (1996) found similar results for air contaminated with emissions from building materials, even though temperature and humidity had no effect, or minimal effect, on the emission rates from the materials. Fanger (1998) observed that people preferred rather dry and cool air, the effect of temperature and humidity being combined as the enthalpy of the air related to the perceived air quality.

Oresczcyn et al. (1999) argue that, when people fill in questionnaires, different interpretations of IAQ may be due to:

(1) Perception or belief that the outdoor air quality is poor.

(2) Lack of air movement.

(3) High air temperature. See Berglund and Cain (1989) and Fang et al. (1996).

(4) High relative humidity. See Berglund and Cain (1989) and Fang et al. (1996).

(5) The combined effect of air movement, temperature and humidity leading to a perception of ‘stuffiness’. (This does not agree with the original definition of stuffiness by Bedford (1964).)

(6) Visual appearance of the work environment (e. g. dirty supply air diffusers).

(7) Perception of poor IAQ used as a surrogate for poor working conditions or management, or for their lack of control over their job, workplace and/or environment.

Standard solutions to such expressed views are to increase the provision of fresh air or to remove sources of pollution but, as Oreszczyn et al. (1999) say, this may not be dealing with the actual cause, which could be 1, 6 or 7, above.

Oresczcyn et al. (1999) reported the results of a study of eight air conditioned buildings and found no correlation between outside air ventilation rate and perceived indoor air quality. Parine (1996) and Bluyssen et al. (1995) obtained similar results.

Posted in Engineering Fifth Edition