Retired air tightness technician Trevor Clark talks about indoor and outdoor air quality and its effects in his monthly column.

Air quality is a phrase used to describe the concentrations of specific pollutants within the air that we breathe, and allows us to describe the air quality within a particular location.

An air quality index (AQI) is used by government agencies to communicate to the public how polluted the air currently is as an external measurement or how polluted it is forecast to become.

Computation of the AQI requires an air pollutant concentration over a specified averaging period, obtained from an air monitor or model. Taken together, concentration and time represent the dose of the air pollutant.

The index can range from good (between zero and 50) to hazardous (over 251).

Health effects corresponding to a given dose are established by epidemiological research. Air pollutants vary in potency, and the function used to convert from air pollutant concentration to AQI varies by pollutant.

Its air quality index values are typically grouped into ranges. Each range is assigned a descriptor, a colour code, and a standardised public health advisory.

On a day when the AQI is predicted to be elevated due to fine particle pollution, an agency or public health organisation might advise people to avoid an area or activity or to wear masks.

Most air contaminants do not have an associated AQI. Many countries monitor ground-level ozone, particulates, sulphur dioxide, carbon monoxide and nitrogen dioxide, and calculate air quality indices for these pollutants.

A website allowing government agencies anywhere in the world to submit their real-time air monitoring data for display using a common definition of the air quality index has recently become available.

Indoor air quality is the air quality within and around buildings and structures.

It’s known to affect the health, comfort, and well-being of building occupants. Poor indoor air quality has been linked to, reduced productivity, and impaired learning in schools.

Common pollutants of indoor air include second-hand tobacco smoke, air pollutants from indoor combustion, radon, moulds and other allergens, carbon monoxide, asbestos fibres and carbon dioxide.

Source control, filtration, and the use of ventilation to dilute contaminants are the primary methods for improving indoor air quality in most buildings.

Determination of indoor air quality involves the collection of air samples, monitoring human exposure to pollutants, collection of samples on building surfaces, and computer modelling of air flow inside buildings.

Indoor air quality is part of indoor environmental quality, which includes indoor air quality as well as other physical and psychological aspects of life indoors, for example lighting, visual quality, acoustics (noise), and thermal (comfort).

Indoor air pollution is a major health hazard in developing countries and is commonly referred to as ‘household air pollution’ in that context.

It is mostly relating to cooking and heating methods by burning biomass fuel, in the form of wood, charcoal, dung, and crop residue, in indoor environments that lack proper ventilation.

Millions of people, primarily women and children face serious health risks. In total, about three billion people in developing countries are affected by this problem.