Indoor Air Quality in Buildings: Causes and Controls

Indoor air quality describes how clean, fresh and comfortable the air inside a building is, judged by the pollutants it carries and by its temperature, humidity and movement. Most people spend the large majority of their time indoors, at home, at work and in transit, so the air in buildings has a bigger effect on daily exposure than the air outside, even in a city.

This article explains what affects the air in a building, why ventilation sits at the heart of the problem, how buildings in Singapore are guided on the subject, and what owners can practically do in homes and workplaces. It treats the topic as building physics first, because most air quality problems are caused, and solved, by how a building is designed, operated and maintained.

What indoor air quality means

Air quality indoors has two broad dimensions. The first is contaminants: gases, particles and biological matter that may cause discomfort or harm. The second is thermal and physical conditions: temperature, humidity and air movement, which do not poison anyone but strongly affect comfort, perceived freshness and the conditions in which mould and dust mites thrive.

Good indoor air quality is often defined in terms of both: air without contaminants at harmful concentrations, and with which a substantial majority of occupants express no dissatisfaction. That second test matters, because complaints of stuffiness, odour or drowsiness are often the first sign that ventilation is falling short, long before any measurement is taken.

Common indoor pollutants and their sources

Indoor air carries a mixture of pollutants from people, materials, activities and the outdoors.

Carbon dioxide (CO₂). Exhaled by occupants. At typical indoor levels CO₂ is not itself the main hazard, but because it rises predictably with occupancy and falls with ventilation, it is widely used as an indicator of whether a space is receiving enough fresh air.
Volatile organic compounds (VOCs) and formaldehyde. Released from paints, adhesives, sealants, engineered timber, new furniture, carpets and cleaning products. Emissions are typically highest after renovation and rise with temperature.
Particulate matter. Fine particles from outdoor traffic and regional haze, cooking, candles and incense, and resuspended dust.
Biological contaminants. Mould spores, bacteria and dust mite allergens, all of which flourish where surfaces or materials stay damp.
Combustion gases. Carbon monoxide and nitrogen dioxide from gas cooking or vehicle exhaust drawn in from car parks and loading bays.
Ozone. Mainly from outdoors, though some office equipment and certain so-called air purifiers can generate it.

Ventilation: the main lever

For most pollutants generated indoors, the primary control is dilution: replacing indoor air with cleaner outdoor air at a sufficient rate. The physics is simple. A pollutant released at a steady rate into a room builds up until the amount removed by ventilation equals the amount being generated. Double the fresh air supply and, broadly, the steady-state concentration of an indoor-generated pollutant halves.

In naturally ventilated buildings, dilution depends on open windows and wind, which is why the design principles in our guide to natural ventilation bear directly on air quality. In mechanically ventilated and air-conditioned buildings, the fresh air rate is set by the system design and can be measured, controlled and, unfortunately, reduced to save energy. Filtration complements dilution by removing particles from incoming and recirculated air, and source control, choosing low-emission materials and extracting pollutants where they are produced, reduces what has to be diluted in the first place.

A point often missed in homes: a typical wall-mounted split air-conditioner recirculates and cools room air but does not bring in outdoor air. A closed bedroom cooled by a split unit overnight therefore relies on leakage alone for fresh air, and CO₂ and other occupant-related pollutants can build up. Opening a window briefly, leaving a door ajar, or providing a small amount of mechanical fresh air are the usual remedies. Our article on aircon installation in Singapore covers the practical side of residential systems.

Indoor air quality guidance in Singapore

Singapore has guidelines and standards rather than a single statute setting indoor air limits for every building. Two documents are central.

The first is the Guidelines for Good Indoor Air Quality in Office Premises, first published in October 1996 by the Institute of Environmental Epidemiology under what was then the Ministry of the Environment, and associated today with the National Environment Agency. It applies to air-conditioned office premises and sets guideline values for contaminants and physical parameters. In that edition, the recommended ranges for acceptable indoor air quality included an air temperature of 22.5 to 25.5°C, relative humidity below 70% and air movement below 0.25 m/s, with an eight-hour guideline for carbon dioxide of 1,000 ppm, alongside limits for carbon monoxide, formaldehyde, ozone and other contaminants.

The second is SS 554, the Singapore Standard code of practice for indoor air quality in air-conditioned buildings. Its current edition, SS 554:2026, covers enclosed air-conditioned premises but excludes residential premises, factory production areas, hospitals, polyclinics and laboratories. It sets IAQ parameters and limits for common contaminants and is the standard building owners and designers refer to for air-conditioned non-residential spaces. Because values can change between editions, designers should work from the current text rather than older figures. BCA’s Green Mark scheme also rewards measures relating to indoor environmental quality.

Humidity, mould and the tropical dimension

In a humid climate, moisture is the indoor air quality issue that most often shows up as visible damage. Outdoor air in Singapore is humid throughout the year, and wherever that air meets a surface cooled below its dew point, by an air-conditioning outlet, a cold duct or a chilled wall, condensation can form. Persistent dampness feeds mould, and mould releases spores and odours into the air.

Air-conditioning helps by removing moisture as well as heat, but only if the system is sized and run to do so, and only if humid outdoor air is not leaking uncontrolled into the cooled space. In homes without central systems, the targeted measures in our guide to dehumidifiers and humidity control can keep problem rooms dry. Where mould has already appeared, our article on mould on walls explains how to trace the moisture source rather than just clean the surface.

Practical steps for better air indoors

Most improvements are straightforward and come down to source control, ventilation and maintenance.

Ventilate after renovation. Air out new rooms thoroughly before occupying them, and choose low-VOC paints, adhesives and boards where possible.
Extract at source. Use kitchen hoods and bathroom exhaust fans that discharge outdoors while cooking or showering.
Maintain air-conditioning. Clean filters, coils and drain trays regularly; a neglected unit can itself become a source of mould and odour.
Manage moisture. Fix leaks promptly and keep damp-prone rooms dry.
Respond to haze. During regional haze episodes, keep windows closed and consider filtration, then ventilate again once outdoor air improves.
Measure when in doubt. A CO₂ monitor is a simple, informative check on whether a room is adequately ventilated.

Frequently asked questions

What is indoor air quality?

Indoor air quality is the condition of the air inside a building, judged by the concentration of pollutants such as gases, particles and mould spores and by physical conditions such as temperature, humidity and air movement that affect comfort and health.

What is a good CO₂ level indoors?

Singapore’s 1996 guidelines for air-conditioned offices set an eight-hour guideline of 1,000 ppm for carbon dioxide, and while CO₂ at such levels is mainly an indicator of ventilation rather than a hazard in itself, readings consistently well above that suggest a space needs more fresh air.

Is there a law on indoor air quality in Singapore?

Indoor air quality in Singapore is guided mainly by NEA’s guidelines for office premises and by the SS 554 code of practice for air-conditioned buildings, which designers and building owners refer to, rather than by a single statute setting limits for all buildings, so the applicable requirements depend on the building type and project.

Do air purifiers solve poor indoor air quality?

A purifier with a suitable filter can reduce particles such as dust and haze, but it does not supply fresh air or remove carbon dioxide, so it complements rather than replaces ventilation and source control, and devices that generate ozone should be avoided.

Why does my bedroom feel stuffy with the air-conditioner on all night?

Most split air-conditioners recirculate room air without bringing in outdoor air, so in a closed bedroom carbon dioxide and humidity from occupants build up overnight, and a little fresh air from a window, door gap or ventilation device usually resolves it.

Investigating damp, mould or air quality problems linked to a building defect? Talk to our consultancy or get in touch.