Interpreting air quality
Standards and objectives
Standards and objectives are numerical benchmarks that help manage short or long-term air quality issues at a local, regional or national level.
Based on scientific information about the impacts of pollutants, they are set at levels that safeguard human health and the natural environment.
Standards are usually legally binding and often include a timeframe for meeting the standard. Objectives (also called goals) are target values to be achieved over the long term.
How are standards and objectives set?
Setting air quality standards and objectives usually involves a risk assessment approach having the following stages:
- hazard identification—What health or environmental impacts will the pollutant have?
- dose–response assessment—How will health or environmental impacts vary with different concentrations of the pollutants?
- exposure assessment—How many people or ecosystems are exposed to the pollutant?
- risk characterisation—How many people or ecosystems are going to suffer specified health or environmental impacts at specific concentrations?
For some pollutants (including those with chronic impacts), the medical evidence indicates that even short-term exposures have acute detrimental effects on people's health, particularly children, the elderly and those with existing medical conditions.
In these vulnerable populations, an adverse response to an air pollutant may occur even when the concentration is significantly lower than those that affect the majority of the population.
Standards and objectives may also take account of technical, social, economic, political, legislative and cultural factors.
Research continues to identify the health impacts of both long-term and short-term exposures to varying concentrations of different pollutants.
This improved knowledge may require changes to air quality standards and objectives and other regulations in the future.
Why do standards have averaging periods?
Standards need to take into account different exposures that may result in different health effects—for example, short-term (acute) or long-term (chronic) effects.
Short-term exposure to raised levels may cause immediate health effects like wheezing or shortness of breath in vulnerable groups, such as asthmatics. In these cases, standards are set for short averaging periods, such as 1, 4 or 8 hours.
These short-term standards are appropriate if the human body can effectively deal with elevated maximum concentrations over shorter exposure times without either irreversible damage or permanent accumulation of the pollutant in the body.
For some pollutants, research indicates their health impact is chronic due to prolonged exposure.
These exposures may, over time, cause health impacts such as reduced lung function, organ damage and cancers.
Because the effects take a longer time to appear, the standard will have a long averaging period (1 year).
Sometimes, we may notice the presence of pollutants even if they are at low levels—for example, odours and irritation—and standards or guidelines are set to account for this.
Air quality exceedences
An exceedence occurs when pollutant levels are higher than the air quality standard or objective target value.
Air quality goals may allow for a certain number of exceedences in a given timeframe to account for natural events, such as bushfires and dust storms, extreme meteorological events, or the time necessary to meet tighter pollution controls.
For instance, the National Environment Protection (Ambient Air Quality) Measure 1-hour goal for ozone is 0.10 parts per million, not to be exceeded more than one day per year.
The graph shows the number of exceedences per year between 1978 and 2011 in South East Queensland.
Limits of detection
Although the sensitivity of measurement methods continues to improve, the level of some chemicals that may be harmful to health can still fall below our current ability to measure them—the limit of detection.
Values below this limit may be stated as 'not detectable’. While literally correct, this does not necessarily mean the pollutant is not present, but only that it is not detectable by the method being used.
If the limit of detection is higher than the standard or objective, then it is not possible to determine if ‘not detectable’ pollutant levels exceed the standard or objective.
However, if the limit of detection is lower than the standard or objective, then ‘not detectable’ pollutant levels will be below the standard or objective.