Monday, May 26, 2008

Cooling machines and their health effects

In India, the urban middle class primarily uses two appliances, desert coolers and air conditioners, for cooling the interiors in the summer and monsoon seasons.

The weather in summers is quite hot in the sea-level regions of India. In North India, the states of Punjab, Haryna and Delhi have temperatures as high as 48 degrees Celsius. The relative humidity can be as high as 80. In south Indian coastal regions, humidity is consistently high and can quickly lead to heat exhaustion.

A combined index called Humidex takes into account both the relative humidity and the temperature to calculate values suitable for humans. Humidex is used by Canadian meteorologists whereas a related index, the Heat Index, is used by US meteorologists. The calculation of the Heat Index uses the Dew Point for humidity rather than the relative humidity.

According to some official studies, a Humidex value of 20 to 30 is considered comfortable, with a value between 30 and 40 indicating mild discomfort. Values above 45 are considered dangerous. Since Humidex is calculated from two variables, a single Humidex value can have multiple possible temperature and humidity values. For example, a Humidex value of 25, for example, can either be temperature/humidity values of 22/60 or 24/40.



If we take the average humidity in June in Delhi as 60, and the average temperature as 40 degrees (this is from 2007 data), the Humidex value is 57, indicating imminent heat stroke. The values are more comfortable indoors, but even a Humidex value of 50 is considered extremely dangerous by the studies.

No wonder anyone who can afford it goes in for cooling appliances. Before 70s, only ceiling fans were being used widely in India. 70s and 80s can be considered the era of evaporative or desert coolers, with ACs becoming affordable in early 90s for a sizable section of Indian middle class (no doubt helped by the higher incomes and availability of credit). Desert coolers are still widely used by lower middle classes. The initial and running cost of a desert cooler is only one fifth of that of an air conditioner.

Desert Coolers

Desert coolers use wet pads and a blower fan in an enclosed box. The pads are kept wet by a small pump which pumps water from a tub attached to the blower box. In dry heat, desert coolers are quite effective in cooling the interiors but when the relative humidity is high, their effectiveness diminishes greatly and they may even cause swamp-like suffocating conditions inside the room.



Compared to the air conditioners, they use fresh air from the outside and do not circulate stale air already inside the room.

But their effectiveness greatly diminishes when the air is already humid. High humidity rates makes the skin and eyes wet, and can cause problems in the lungs. The air supplied by the coolers is typically of 80-90% relative humidity which reduces the evaporation rate of the body. Since the desert cooler wet pads are not dense enough to filter air completely, pollen, various microorganisms, contaminants can be blown in by the cooler fan. Certain bacteria and mosquitoes breed more effectively in humid atmospheres (as also in the water tub of the cooler) and can cause significant health risks.

Air Conditioners

Air conditioners cool air as well as remove humidity. Simplistically speaking, they do it by passing the air through coils filled with compressed gas. The gas expands by taking heat away from the air, and a blower sends the cooled air into the room.

The coils are cooler than the environment and as such, they take moisture away from the air due to condensation. Unless a humidifier is built into the air conditioning unit, the air inside a normally air-conditioned room is very dry.



Most normal air conditioners sold as window ACs or split ACs have no humidity control or humidifier units. Expensive unit, not available in India, route the condensation and heat back into the room to control humidity.

Air conditioners usually have filters which clean the air as it passes through it. If the filters or the inside of an air conditioner is dirty, it can introduce various kinds of mold and microorganisms in the cool air which can exacerbate asthmatic attacks and cause ill-health.

Low levels of humidity can cause dryness of skin and eyes, irritation in the throat and lungs, headaches and overall discomforts. At humidity levels of 30, the minimum comfortable temperature is 27. Prolonged exposure to low-temperature low-humidity environment will lead to respiratory and skin discomfort.

Many low-end air conditioners also lead to a lack of fresh air and oxygen in the room. It is easy to smell the stale air in a room after an overnight run of a non-vented air conditioner. Split ACs especially have no access to fresh air and only cool the inside air and remove humidity from the room. Split units should only be installed in showrooms and high door opening frequency areas.

Conclusions

The air inside a house should be reasonably cool, reasonably moist, reasonably clean and reasonably fresh. Air coolers provide a lot of fresh air, but can cause excessive humidity. Window air conditioners remove moisture from the room. When using an AC without a built-in humidifier, the ideal setting is to have a temperature setting of 27 and above, with open vents to allow in fresh air. Split air conditioners should be avoided in home settings.

In case one is using a desert cooler, a dehumidifier unit (not easily available in India) can be used. If a dehumidifier is not available, desert coolers should not be used in high-humidity conditions.

In case one is using a window AC and the air is getting too dry for comfort, a humidifier unit (or a toy fountain etc.) can be used to introduce humidity in the room. In case no humidifier is easily available, one can make one's own by having a pan of water over a portable electrical hot plate. The water vapor will not let humidity in the room go down to dangerous levels.

If the room is not big, one can also use a portable steam inhaler (easily available for Rs 150 in a roadside chemist shop). The only downside to using an inhaler as a humidifier is that the content of water is very limited and it may only work for a few hours. The market may be ripe for a portable humidifier costing a few hundred rupees which will come in very handy in winter months as well when humidity is very low and people use room heaters and heat convectors.

In any case, the various pads, filters and internal casing of the cooling unit must be periodically cleaned to avoid fungus/mold, bacteria and dust.

If the air conditioner does not have a vent for fresh air, leaving open a small window is a very good idea.

References

http://en.wikipedia.org/wiki/Air_conditioner

http://en.wikipedia.org/wiki/Air_cooler

http://en.wikipedia.org/wiki/Humidex

http://en.wikipedia.org/wiki/Heat_index

http://www.ccohs.ca/oshanswers/phys_agents/humidex.html

http://www.mouthshut.com/review/General_Advice_on_Air_Conditioners-131230-1.html

http://www.wunderground.com/history/airport/VIDP/2007/6/25/MonthlyHistory.html?MR=1

http://en.wikipedia.org/wiki/Humidifier

3 comments:

rmdseo said...

thanks for sharing this wonderful post

D Kessal said...

Really great article ... God bless you !

Anuj gupta said...

Very interesting and informative.