Poor Indoor Air Quality can arise from many types of air contaminants arising from many sources. In office buildings, they can be classified into three groups, that are gases and vapours, such as CO, from people, butyric acid from roads and highways, CO from adjacent parking lots, tobacco smoke, VOCs from paints, wood panelling, office equipment, formaldehyde from carpeting and furnishings. Another source is inert particles like dust and man made fibres and thirdly micro-organisms like fungus, bacteria, virus and molds resulting from high humidity and damp corners.

Inadequate maintenance of the HVAC system, can become a breeding ground for most of the pollutants and inadequate ventilation leads to, a concentration of pollutants indoors.

There can be two basic solutions to improving the indoor air quality.
•Source Control
•Pollutants Removal
   a. Ventilation
   b. Air Cleaning

Source Control, though a preferred approach, may not often be practical. Source control measures are pollutant specific and may include use of low formaldehyde emitting materials, banning of cigarette smoking, prevention of radon entry through sealing of foundations, eliminating use of asbestos and storing of paints and chemicals outside the occupied space. Controlling of relative humidity will prevent the growth of mold, mildew and microbial contaminations. Routine or regular maintenance of HVAC system
is another method of reducing the pollutant level.

Air cleaning is the second method of removal of pollutants. It is not a substitute for fresh outside air. Air filters, ionizers and electrostatic precipitators can effectively rid indoor air of some microscopic particles such as dust and some smoke, but they do not totally purify and revitalize the air. These devices must be sized and maintained properly in order to work effectively.

An advantage of some air cleaners is their ability to remove some particles such as pollen, that originate outside. A disadvantage is that they have no effect at all on the concentration of some pollutants such as formaldehyde, carbon monoxide and other gases.

Ventilation is the natural movement of air through a building. Removal of contaminants from a building or reducing its concentration within a work space can be accomplished by either passive or active ventilation.

Passive ventilation refers to infiltration / exfiltration or the air exchanged through doors, windows or other openings by natural forces. In most airconditioned buildings, these openings have been reduced to the minimum to conserve energy. Active ventilation systems provide continuous ventilation through windows. Regular ventilation not only provides a sufficient supply of oxygen but unavoidable pollutants, contaminants and unpleasant odours.

Sufficient ventilation, especially in summer can prevent excessive humidity. In places where ventilation through the windows is not always possible, mechanical ventilation is used.

Mechanical ventilation is the airflow out of and into a building resulting from pressure differences caused by fans or blowers.

Unfortunately, what we call, fresh outdoor air is not so fresh any more, but quite polluted. Under such circumstances, ventilation though the most important method alone will not suffice as the main measure for improving indoor air, but a combination of source control, filtration, air cleaning and ventilation provides the ideal mix for improving indoor air quality.

"The Solution to Pollution is Dilution"

This is an age old dictum, the practice of which was given up by building owners in the wake of energy conservation, by making tighter buildings.

Dilution refers to ventilation with outdoor air, to reduce the concentration of pollutants in a room or building. However, in recent times with growing indoor air quality problems and its effect on the health of the occupants, it was discovered that the ventilation standards
designed to dilute the pollutants were found to be insufficient. Air enters a building in three ways infiltration, natural ventilation and mechanical ventilation. Outdoor air mixes with indoor air and reduces the concentration of pollutants.

The rate at which outside air replaces indoor air is described as the air exchange rate. When there is little infiltration, natural ventilation or mechanical ventilation, the air exchange rate is low and pollutant levels can increase.

In fact as the figure overleaf shows, the dilution principle says that the concentration of pollution is inversely proportional to the ventilation rate.

The air exchange rate of average buildings today is 2 air changes per hour. In tight buildings air changes per hour is as low as .2 to .3. In leaky buildings it can be as much as 3 air changes per hour.

It is important to understand that an air exchange rate of 1 air exchange per hour does not mean that all pollutants will be removed is one hour, but that with 2 to 3 air changes per hour the concentration of pollutants will be low.

Ventilation is a process of dilution and removal that gradually gets rid of pollutants. In addition, pollutant removal is further slowed down by the fact that some pollutants are trapped by carpets, drapes and other surfaces only to be released into the air later.

An existing ventilation system which is inadequate because of design flows, poor maintenance or expanded use of a building will often be associated with poor indoor air quality. Redesign or maintenance of HVAC system may be required.

The remedy for IAQ problems thus becomes obvious - Increase Ventilation

Increased Ventilation Standard vs. Energy Management

The Challenge !

As the recommended levels of outside air brought into the conditioned space has been increased by 3 times (to 25.5 CMH (15 CFM) from 8.5 CMH (5 CFM) per person) much higher latent and sensible loads are imposed on the cooling and heating equipment.

Introduction of even a small quantity of air into an HVAC system raises physical plant requirements dramatically, creating a need for balancing energy needs with the IAQ standard.

The challenge before the designers and contractors, is to design HVAC systems in buildings, restaurants and hospitals tor an improved indoor environment at no extra utility bills for the owners.

A challenge yes! but impossible - No! with options for effective energy recovery equipment available.

In the HVAC industry, ASHRAE American Society of Heating, Refrigeration and Ventilating Engineers) plays a leadership role in research and setting standards.

The ASHRAE standard for acceptable indoor air quality specifiesminimum ventilation rates that will be acceptable to human occupants and are intended to avoid adverse health effects. The standard 62-1999 recommends fresh air intake of 25.5 TO 34 CMH (15 to 20 CFM) per person where 8.5 CMH (5 CFM) was considered adequate by the industry. CO2 levels, which have been recognized by ASHRAE as the surrogate ventilation index (being the only economically and practically measurable variable) should not exceed 1000 ppm.