Post 528 – by Gautam Shah
An interior space is finite and a well-controlled entity. It is a safe place against many of the threats to survival, but for the quality of air. The finite volume depends on outside environment for dilution of contaminants that are generated internally. It depends on controllable openings to filter out the external fouling elements. The interior space is a finite and predictable place and so both the processes can be managed. Indoor air pollution is often 2 to 5 times as bad as outdoor pollution. In few cases it is as worse as 100 times than the outdoors. Same holds true for Humidity, indoor humidity levels are higher then outdoors, except during the rain or just after a downpour, when it can be more for a while. When both, the internal and external environments are not appropriate, one has to rely on electro-mechanical or chemical filters or scrubbers.
One of the best and passive way is to continuously refresh the interior environment with the exterior air. Nominally an exterior air has proportionately lesser pollutants due to greater volume and dilution. But that may be highly contaminated at certain times, seasons and places. One of the major fouling factors of external environment is the street level condition. High speed moving traffic, even where non-fossil fuels are used, raises the dust level with air turbulence. Street level doors such as entrance foyers are used as escape points for used air, to reduce the casual intake of street level contaminants.
An interior space is perceived to be safe whenever there are excessive external pollution and associated discomfort. The interior space, however, has a finite volume and can support human occupation for a while, before it gets fouled up. One has to depend on dilution exchange with external environment. The external environment can be locally conditioned with use of elements such as green foliage or plants, water bodies, built baffles, better oriented and comparatively taller intake openings.
Passive air movement is one of the best means of diluting the interior pollutants. For air movements, the human settlements are planned with macro wind-breeze movements over the terrain. But, when air movements within the buildings are considered, the scenario changes substantially. The micro air currents that help interior exterior exchange are chiefly governed by pressure-temperature differential across a profile of set of openings. The local pressure-temperature gradients arise due to the colour and texture of surfaces, shapes and sizes of architectural elements, size, shape and level of openings, and shadowing effect of projections. A water body or greenery outside an intake opening adds moisture and cools the temperature of intake air. Similarly a hard and dark surface on outward opening accelerates the ventilation effect.
Passive air exchange works due to the difference across the exterior and interior air pressures and temperatures. In tropical areas and on warm days the exterior air replaces the interior air, but with few disadvantages. Exterior air is drier and can remove the interior moisture. It is warmer and so warm up the interior temperatures, and the high pressure air current can cause irritating skin sensation and affect the elders’ and infants’ body temperature mechanism. The air exchange can also advantageously reduce the high proportion of moisture, the temperature, and offer skin-feel comfort. For colder climes and colder days the air exchange through openings or leakages can decrease the room temperature, evacuate the moisture and cause localized body cooling through air droughts.
The interior air gets contaminated due to various activities of human (and animals) occupation, and in many instances even without it continues to get contaminated. Building materials, finishes, furnishings generate high volume of pollutants in their latent state. These are in the form of unwanted gases, moisture, volatile organic compounds (VOCs), odours, particulate matters, etc.
Indoor air quality control is achieved with reduction of volatile organic compounds (VOCs), and microbial impurities. Buildings depend on passive and mechanically powered means to achieve adequate exchange with cleaner air of the outdoors. Screen filters, scrubbers (water, air and active solid particles) and methods of isolation, are chief means of dilution. Proper selection of building materials, surface finishes, furnishing materials, and the maintenance techniques and products go a long way in upgrading the interior air quality. The degradation of building materials and residual products of maintenance increases several times when the surface temperatures are high such as near stoves, heating elements, thin body materials exposed to solar heating.
High volume rooms (such as with higher ceilings) are preferred in tropical dwellings, because these offer greater volume per (person) occupancy and so higher dilution of contaminants. The high volume also offers greater internal air movement or turbulence for better mixing of fresh air with contaminated air. Low volume rooms (such as with lower or barely adequate ceilings heights) are found in modern multistoried apartments and air-conditioned offices. Such low volume spaces economize on cooling-heating costs, but these are not working or adequately operational the quality of air deteriorates. The mixing effect due to turbulence is not able to ventilate all pockets, such as undersides of desks and cabinets. Such spaces have stagnant, moist and warm air, breeding ground for mosquitoes. Spraying mosquito repellents or insecticides only adds to the pollution.