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Indoor air in Conditioned spaces

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At the time when the media is filled with reports of pollution levels in the air especially in Delhi, it may appear to be strange to be concerned with quality of indoor air quality (IAQ). Here is why we should be concerned.

Today at any given point of time 70% of the urban population is spending their waking hours in an enclosed environment – from offices and multiplexes to hotels and restaurants. The outdoor air which is highly polluted is pulled indoors in conditioned spaces. The pollution builds up indoors as the air is recirculated to maintain the temperature.

Over 80% of our lives are spent in the indoor conditioned spaces where the air can be six times more polluted than outdoor air. As a first step, let us first identify the pollutants in outdoor & indoor air and their sources.











The Outdoor Air Pollutants

Outdoor air pollutants are of two types: Particulate and Gaseous.


Source – Soot, particulate contaminants, energy production (especially coal power Plants), ‘chullahs’, open fire cooking, stubble burning, burning of waste, industrial process emissions, etc.

Effect on Health

Increasing respiratory symptoms, e.g. irritation of the airways, coughing, or difficulty in breathing; decreasing lung function; aggravation of asthma; development of chronic bronchitis; adverse effects on the cardiovascular system; and premature death of people with heart or lung diseases.

Acceptable levels

Particulate matter (PM)                         2.5 24h mean 3 < 25μg/m

Particulate matter (PM)                         10 24h mean 3 < 50μg/m

How do we enhance IAQ?

As building owners, architects, HVAC consultants, facility managers, we can improve the quality of indoor air especially in schools, offices and residences by designing and maintaining the conditioned spaces so that contaminants are limited to the acceptable levels. The best solution to address indoor air pollution is dilution using outside clean air.

Flush out indoor pollutants using clean outside air and prevent infiltration by positively pressurizing the area. To clean out side air, proper particulate and chemical filtration is required. In addition, ventilation removes or dilutes the pollutants, stale odors and brings in more fresh air in the conditioned space. However, bringing in more fresh air means more air-conditioning tonnage and more energy!

Thus, providing the necessary fresh air for the building occupants, present a tough challenge to the HVAC engineer. The outdoor air at a higher design level needs to be conditioned to the level of the indoor design condition, which increases the air conditioning tonnage considerably. The recurring energy expense is another matter of concern. A ventilating device equipped with an efficient energy recovery wheel offers the suitable solution. The air, exhausted for bringing in fresh air, takes with it, the energy used to condition it. The energy, so lost otherwise, can be recovered with the aid of energy recovery wheels and used to pre-condition incoming fresh air and thus save energy. The air purifying equipment design should allow for customization depending on:

City / Area / Application

Outdoor Air Pollution Level

Harmful Gases

Source – Vehicular traffic, industrial processes, open sewage systems

Effect on Health

Sox irritates the throat and lungs. If there are fine dust particles in the air, it can damage a person’s respiratory system; and if combined with other substances in the air, it produces a haze that reduces visibility.

NOx causes increased incidence of respiratory illness, increased airway resistance (due to inflammation), damage to lung tissue, chronic obstructive pulmonary disease or COPD (narrowing of the airways), emphysema (as part of COPD), pulmonary edema (accumulation of excessive fluid in the lungs), infant and cardiovascular death.

Acceptable levels

Sulphur dioxide (SO2)                  24h mean 3 < 20 μg/m3

Nitrogen dioxide (NO2)                1h mean 3 < 200 μg/m3

The Indoor Air pollutants

VOCs; Formaldehyde; Radon asbestos; Virus & Bacteria; Mould, Mildew & Fungus.

Source – Printers, photocopiers, fax machines; carpets, cleaning products; paints, polish, solvents; moisture and people themselves.

Effect on Health

Short Term Exposure (few hours or days) cause irritation of eyes, nose and throat, Asthma attacks, acute bronchitis, increased susceptibility to respiratory infections and lung disease. Long-term Exposure (Years) can reduce lung function, develop chronic bronchitis and even cause premature death.

Acceptable levels

Carbon dioxide (CO2)                  < 1000ppm

Ozone (O2)                                     8 h mean 3 < 100 μg/m3

Carbonmonoxide (CO)                 8h mean < 75ppm

Formaldehyde                                < 0.1mg/m3

Vertical or Horizontal Units (Space Limitations)

Retrofit Cases

The air purification system should incorporate particulate filters, gas phase filters, odour filters

Along with a total energy recovery, device should offer option for controls, wet scrubber and evaporative cooling. It is equally important that the ventilating device is not only able to clean the outdoor air but also not raise the energy cost.

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