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Air quality can be measured, and the measures are regularly published in global cities. But this does not happen to the same degree in South Africa, even though the data are available. The data for Johannesburg are alarming and sufficiently dangerous to trigger a host of respiratory and cardiovascular diseases as well as cancers.

The most common measure is the air quality index (AQI), a measurement system used to translate levels of air pollution into a simple number that indicates the potential health risk posed by the air people are breathing. Instead of reporting only the concentration of individual pollutants, such as the amount of fine particulate matter in the air, the AQI converts these measurements into a scale, where higher numbers represent increasingly dangerous conditions.

An AQI reading between 0 and 50 is considered good, meaning air pollution poses little or no risk. Between 51 and 100 is classified as moderate, where air quality is generally acceptable, but some sensitive individuals may experience effects. Between 101 and 150 is considered unhealthy for sensitive groups, while readings above 150 are classified as unhealthy, meaning the general population may begin experiencing health effects. Above 200 is considered very unhealthy, and readings above 300 are classified as hazardous, where emergency health warnings may be issued.

The AQI is not a direct measurement of pollution itself. Instead, it is calculated from measurements of several pollutants, with the pollutant posing the greatest health risk determining the final AQI score. The main pollutants measured are physical particles floating in the air, ozone, nitrogen dioxide, and sulphur dioxide.

The most important measure relates to very fine particles. These are seen as particularly dangerous because they are small enough to penetrate deep into the lungs and enter the bloodstream. They are produced by sources such as coal burning, industrial activity, vehicle emissions, household fuel burning, and wildfires.

Air quality data are collected through satellites and monitoring stations equipped with sensors that measure pollutant concentrations.

Johannesburg’s air quality is highly seasonal. While the city’s annual average pollution levels are often classified as moderate, this masks significant winter pollution episodes when air quality deteriorates sharply. Johannesburg’s annual average corresponds to an AQI of roughly 66, which is more than three times higher than the World Health Organisation’s recommended annual guideline.

The typical Johannesburg year follows a clear pattern. During summer, from December to February, air quality is generally better, with AQI readings often ranging between 30 and 70, because rainfall removes pollutants from the atmosphere and stronger atmospheric mixing disperses pollution. During autumn, from March to May, pollution levels begin rising. Winter, from June to August, is the worst period, with AQI readings frequently reaching very unhealthy levels of between 100 and 200 or higher. During spring, from September to November, pollution gradually improves as weather conditions become more favourable.

Johannesburg’s winter readings would typically trigger public health warnings in many countries as likely to cause negative health effects.

The reason Johannesburg experiences its worst pollution during winter is because of its location on the Highveld. During the colder months, atmospheric conditions trap pollution close to the ground. Cold air settles near the surface, reducing the movement of air and preventing pollutants from dispersing. At the same time, there is little rainfall to wash particles out of the atmosphere, while increased household fuel burning for heating adds further pollution. Industrial activity, vehicle emissions, and coal-fired power generation also contribute to the winter build-up.

This creates frequent temperature inversions, where a layer of warmer air traps colder polluted air below it. The result is that pollution can accumulate over Johannesburg and surrounding areas for days at a time.

The challenge for Johannesburg is that these pollution levels have become normalised and residents live with the experience of poor air quality as part of everyday life. A day with an AQI of 150 or higher, which would attract significant public attention in many developed countries, may be the permanent condition of the air Johannesburg residents breathe for months at a time.

The most meaningful problem for Johannesburg is therefore not a single AQI reading on a particular day, but rather the effects of persistent long-term exposure to toxic air.

The most immediate and significant health impacts are on the respiratory system and translate into chronic respiratory diseases such as chronic obstructive pulmonary disease, chronic bronchitis, and reduced lung function. Air pollution can worsen asthma by irritating and inflaming the airways, increasing the frequency and severity of attacks. Children are particularly vulnerable because repeated exposure during development can restrict lung growth, leaving them with permanently reduced lung capacity.

The effects extend beyond the lungs.

Particulate pollution is strongly linked to cardiovascular disease because particles can enter the bloodstream and contribute to inflammation, blood vessel damage, and increased strain on the heart. Long-term exposure is associated with higher risks of heart attacks, strokes, high blood pressure, and heart failure. Cardiovascular disease is one of the largest contributors to pollution-related deaths globally, meaning that the danger from polluted air is not only respiratory but affects the entire circulatory system.

Air pollution is also a recognised cancer risk.

The International Agency for Research on Cancer, part of the World Health Organisation, has classified outdoor air pollution as a carcinogen, meaning there is sufficient evidence that it causes cancer. The strongest association is with lung cancer, including among people who have never smoked.

Research has also linked long-term air pollution exposure to neurological effects. Persistent exposure to small particulate pollution has been associated with faster cognitive decline among older adults, increased risks of dementia, and possible effects on brain development in children. Pollution exposure during pregnancy has also been linked to adverse outcomes, including premature birth, low birth weight, and impaired foetal development.

The severity of the health impact depends on the length and frequency of exposure. A short period of AQI 150 conditions may trigger breathing problems or cardiovascular events among vulnerable people, but repeated exposure over years can contribute to chronic disease and reduced life expectancy.