Occupant comfort not only refers to comfort which is affected by temperature, humidity, air movement but also to odours, harmful biological contaminants and chemicals present in the conditioned space. According to D E V S Kiran Kumar, Research Associate at Sustainable Habitat Division, The Energy and Resources Institute, after the advent of green building concept much attention is being given to the indoor environment to create a healthy atmosphere and improve occupant productivity.
Indoor Air Quality (IAQ) in a conditioned area refers to the nature of conditioned air that circulates throughout the space/area and the air one breathes when indoors. Usually in large commercial and public establishments, the occupants do not have a control over the indoor environment to open windows during stuffy and unaired conditions or to put on the exhaust fans when the room is filled with dust, tobacco smoke, pesticide sprays and fine carbon dust from copying machines. It has been observed that the indoor air in conditioned space can be 10 times more polluted than outdoor air due to poor ventilation that may lead to health risks. This presents the necessity of studying IAQ in conditioned spaces.
ASHRAE defines ‘acceptable indoor air quality’ as “Air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80 percent or more) of the people exposed do not express dissatisfaction” To reduce the indoor air pollution there should be proper ventilation systems installed in a building. They should effectively remove hazardous chemicals and particles from the air breathed by individuals to create a healthy indoor environment.
Currently, most of the corporate and commercial buildings are built with glazed facades using centralized air-conditioning systems. These buildings might look elegant but the indoor air circulation is often very poor and could affect the health of the people working inside. They may contract major health problems due to inadequate ventilation and off-gassing of chemicals found in many building materials. WHO reports that indoor air pollution causes 14 times more deaths than outdoor pollution. Therefore, the focus today is not only on our Environment but also our Invironment.
Indoor air contaminants can originate within the building or be drawn in from outdoors. If contaminant sources are not controlled IAQ problems can arise in spite of the properly designed and well-maintained HVAC systems.
The sources of indoor pollution are broadly classified as-
• Sources outside building
• Building components and furnishings
• Other sources
Indoor pollutants have to be identified in a building and proper measures should be taken to limit the pollutants, especially Formaldehyde, VOCs and tobacco smoke. Using low VOC paints and adhesives for indoor finishes and furnishings, allotting a separate zone for smoking outside a building and improving ventilation rate would help for better air quality.
Indoor air is not always as clean and safe, especially in buildings where hazardous chemicals are used or where combustion processes occur. Also, much attention is being given to the effect of the radon, asbestos, cigarette smoke and the terms Sick Building Syndrome (SBS) and Building Related Illness (BRI). To maintain good air quality inside the building it is important to prevent these problems by avoiding the sources of pollution and proper maintaining of different building elements.
Solutions for IAQ problems
Preventing IAQ problems is generally much less expensive than identifying and solving them after they occur. It is practically much easier to halt the less harmful chemical compounds at the source itself. It is required to evaluate the properties of the adhesives, paints and composite wood products being used in the building interiors. Pollutants (both vapour and particle) can often be captured by judiciously located inlet ducts and exhaust fans called local ventilation systems. The commonly used ventilation systems are kitchen hoods, drier vents which are used to capture polluted indoor air and exhaust them to the outdoors.
In industrial settings environmental regulations prohibit the release of certain contaminants into the outdoor air. And such pollutants must be removed from the captured air by devices called air pollution control systems (e.g., filters).
Protection of air handling systems from dust reduces potential for problems inside the building. Filtering the outside air, increasing ventilation rates and dehumidifying air provides better air quality for the occupants. Installation of automatic sensors and controls to maintain proper temperature, humidity and rates of outdoor air introduced to occupied spaces also plays a key role in maintaining optimal air quality (e.g., CO2 sensors).
While designing air-conditioning systems for buildings, refer to fresh air requirement to maintain better indoor air quality. The table below provides the minimum fresh air required in a mechanically ventilated or air conditioned spaces recommend by NBC 2005.
The strategy given by ASHRAE is to dilute the indoor contaminants by ventilation i.e. the process of supplying or removing air by natural or mechanical means to or from the space. Ventilation systems are used to maintain a good thermal comfort level and acceptable IAQ in an indoor environment, at a reasonable cost.
Various technical methods are available for air purification in case of high indoor pollution.
Electrostatic filters incorporate non-woven polypropylene media in which a permanent electrostatic charge is given to the fibres which attract and trap the airborne contaminants. The Electrostatic filter needs to be periodically replaced, which is a recurring cost to the user. These filter media attracts the particles with dipole in a way that the dust collected over the television screen.
2. Activated Carbon Filters
The odours and chemical vapours present in a conditioned space must also be removed to totally clean and purify the air. Activated carbon known as universal adsorbent (i.e., absorption at the surface) of the medium is employed for removal of chemical, organic and human odours. Since its basic function is not filtration, it has to be used in conjunction with electrostatic filters. Activated carbon filters are used in central air-conditioning plant of large spaces such as Hospitals, Mortuaries etc.
3. Negative Ion Generator
Studies have shown that the presence of negative ions in air bring about a feeling of well-being and a significant reduction in the number of complaints of headache, nausea and dizziness associated with air-conditioned environments. Negative Ionisers release a flood of negative ions into the air and refresh the air. These are electrical devices which basically consist of a pin or a metal wire which selectively lends a negative charge to the air stream flowing over the pin, negatively ionizing the particles which are released into the conditioned space.
Electrostatic precipitators are large stand-alone devices used with suitable filtration media in air pollution control system such as in cement plants, spinning yarn processing plant etc. They require high voltage of current to charge and collect the small particles. Due to its constant flow, low pressure drop and efficient filtration of sub-micron smoke and fumes these filters are more popular compared to others devices.