[ARCHIVÉ] Conseils concernant la chaleur: Climatisation mécanique

How Effective Are Portable Air Coolers in the Home?

A review of personal heat protection advice, directed to the general public, found that staying in cool or air-conditioned environments was one of the most common recommendations during extreme heat events. However, these recommendations rarely specify central air conditioning or home window units; they advise spending time in cool places, such as basements or air-conditioned buildings.¹ Dehumidifiers and evaporative coolers are not generally recommended; it is not clear whether this is because they are ineffective or because of a lack of research, evidence, or practicality. 

Air Conditioners

Air conditioners are a common means of cooling indoor air. Recirculated air is passed over cooled coils, which results in air cooling and humidity removal.² They are effective in both humid and dry climates, but in humid regions they offer the additional benefit of increasing comfort (and, greater effectiveness of thermoregulatory mechanisms) through dehumidification.

There is ample evidence showing the effectiveness of air conditioning when protecting against heat-related morbidity and mortality during extreme heat events.^3-12 Although some sources warn against moving between hot outdoor and cool air conditioned environments¹³, there is no evidence to indicate that this causes harm. It is not known how much time spent in air conditioned environments is optimal, but in general, the more air conditioning exposure, the less chance for heat-related effects.³ For occupational settings, health and safety limits (TLV®s) are based on wet bulb globe temperature (WBGT - a weighted average integrating the effects of temperature, air movement, humidity, and infra-red radiation) averaged over 2 hours, type of clothing, and activity level; limits are expressed as percentage of time a worker can be exposed to various combinations of activity and heat.¹⁴ Thus, periodic relief from the heat is considered protective. 

In two studies that looked at central versus room-unit air conditioners, only central units showed a consistent benefit.^{9, 15} O’Neill (2005) examined heat-related mortality risk in four major U.S. cities over seven years and found that for each 10% increase in central air-conditioning prevalence, heat-associated mortality, pooled across all four cities, dropped by 1.4% (95% CI = –0.1 to 2.9), but room units did not show an effect.¹⁵ Rogot (1992) compared deaths in persons with and without air conditioning during hot as opposed to cool months by state across the US. He found that on hot versus cool months, persons with central air conditioning had an odds ratio for death of 0.73 (i.e., a benefit of 1.4) while those with room units had an odds ratio for death of 0.96 (i.e., no significant benefit). However, room-unit air conditioners were significantly beneficial among people in small dwellings with one to three rooms, showing an odds ratio of 0.41 (i.e., a benefit of 2.4).⁹ Declining rates of heat-related mortality among the elderly in the United States have been attributed to increased use of air conditioning.^{7, 8, 16, 17}

Many people locate their room air conditioner in the bedroom, when central air conditioning is absent. No studies have specifically examined the risk of heat-related health effects related to night time air conditioner use, but lack of night-time relief from heat has been blamed for increase in deaths when heat episodes linger for several consecutive days.^{3, 18-20} Particularly in urban areas, buildings and paved areas retain heat during the night; differences in minimum temperatures between urban and rural areas have registered as much as 9°C²¹. In his analysis of heat-related mortality in the US, Rogot et al.⁹ found that, unlike central air conditioning, room air conditioning offered no risk reduction except in dwellings of 1 to 3 rooms. This would not support the conclusion that bedroom-only air conditioning was protective, except in the case of single rooms with doors closed or very small (e.g. studio) apartments.

Dehumidifiers

Humidity has an effect on perceived heat (comfort level) as well as on evaporative cooling (evaporation of sweat). Excess moisture in the air is associated with an increased burden of heat.²² Because dehumidifiers remove moisture from the air, it seems logical that they would be beneficial in minimizing discomfort and preventing heat- related illness. However, research on the effectiveness of dehumidifiers, particularly portable units, in preventing heat illness is lacking.

It should also be noted that dehumidifiers produce heat through the process of condensation. Unlike air conditioners, which are mounted in windows or outdoors, the heat produced by a dehumidifier remains in the room. On the other hand, the dehumidification process involves passing water over a cooling coil to condense water from the air, increasing comfort through humidity control. This raises the question, even though dehumidifiers increase air temperature, can they make sweating more effective, thus increasing comfort and decreasing body temperature? If it were possible to exhaust dehumidifier units to the outdoors, such an effect is certainly plausible.

Evaporative Coolers

Evaporative, or “swamp”, coolers are sometimes used in dry climates to cool hot, dry air through the evaporation of water, thus adding humidity to the airstream.²³ They operate by drawing air through a wet filter material; moisture from the filter evaporates and can cool the air by approximately 5 to 9 °C. They rely on a flow of outdoor air, usually from an open window, in combination with the water-cooled membrane. Evaporative coolers are considered most efficient when outdoor air relative humidity is below 30%, but may be effective at relative humidity up to about 50%.²

Research has suggested that evaporative coolers may be effective even where relative humidity is high, if they operate with sufficient air circulation. (NB Authors of the paper, first published in 1966, do not specify the relative humidity under which evaporative coolers were tested; however, it is clear that it was higher than what was normally considered optimal for use). They emit cool moist air which causes heat loss through convection as it moves over the body; if sufficient heat is lost to prevent sweating, the need for cooling by sweat evaporation is diminished or removed.²⁴

 

Because of their mechanism of action and effect on humidity, evaporative coolers have the potential to introduce health risks associated with airborne organisms or toxins. The humidifying effect of evaporative coolers has been found to support dust mite survival in arid regions in the United States, where mites normally do not thrive.²³ Evaporative coolers in homes are also associated with airborne algae and other microorganisms.²⁵ Asthmatic symptoms have been found to be more frequent in homes with evaporative coolers²⁶, but the relationship between symptoms and contaminants of evaporative coolers is not entirely clear.²⁵

Fans

Fans can help to maintain normal body temperature by increasing air movement and stimulating evaporative heat loss.²² However, fans can also heat the body through convection, if the air is hot, making them less than effective at elevated temperatures (usually defined as above 35°C) and high humidity.²² (For further details, see our upcoming article on Fans.)

In conditions where fan use alone may be inappropriate, air conditioners, or evaporative coolers may cool the air sufficiently to make fans safe and effective. In fact, fans are more effective at cooling individuals when other means are used to reduce temperature and decrease humidity. Research is lacking on the relative effectiveness of different combinations of mechanical cooling methods in the home.

Summary

Getting relief from the heat, whether by cooling the home or going to a cooler location, is an important means of reducing the risk of heat-related illness. For homes without central air conditioning, single-room air conditioning units (with doors closed), evaporative coolers or fans may be beneficial; the potential benefit of room-unit dehumidifiers is less certain. Questions remain about which methods – or combination of methods – are optimal under different conditions of temperature and humidity.

Gaps and questions

• The efficacy of single-room air conditioners varies depending on a range of factors including: unit cooling capacity, room size, ventilation rate (leakage), insulation and thermal mass value of walls, outdoor temperature, humidity, radiant load, etc. A simple metric would be helpful for individuals to calculate minimal and/or optimal conditions for unit choice and placement to prevent heat-related illness.
• How much time should people spend in cool environments on hot days in order to decrease their risk of heat illness (other than “more is better”)?
• Under what conditions, if any, might dehumidifiers be useful in hot weather? Given the fact that they increase air temperature but decrease humidity, to what extent can they increase the body’s own mechanism of cooling by evaporation of sweat? Can an increased ability to sweat compensate for the increase in air temperature from the dehumidifier unit?
• How can fans be used in combination with air conditioners or evaporative coolers to increase safety and efficacy?
• Given the fact that evaporative coolers increase relative humidity, to what extent do they decrease the body’s own mechanism of cooling by evaporation of sweat? Can they cool the body enough to supplant the need to cool through sweating?

 

References

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