[Tony Sousa]

Today the Air Quality Index (AQI) in Johannesburg is 115 meaning :Unhealthy for Sensitive Groups”. What does it mean “unhealthy for sensitive groups”?

The short answer is:

Because it’s winter in Johannesburg, and winter is unfortunately the worst season for air pollution.

The main culprit is usually PM2.5 (very fine particulate matter), which is exactly what my IQAir app is measuring (download from Google Play Store the “IQ Air AirVisual app)

Why is it worse today?

Several factors often combine:

1. Cold winter mornings
   • Many households burn coal, wood, paraffin or other fuels for heating and cooking.
   • This releases large amounts of PM2.5 into the air. (IQAir)
2. Temperature inversion
   • On many Highveld winter mornings, a layer of warmer air sits above cooler air near the ground.
   • This acts like a “lid”, trapping pollution close to the surface instead of allowing it to disperse. (IQAir)
3. Little wind
   • When the wind is light, pollutants remain over Johannesburg instead of being blown away.
4. Traffic
   • Morning rush-hour traffic adds exhaust emissions and fine particles.
5. Industry and power generation
   • Emissions from industries, coal-fired power stations, mining activities and dust all contribute to background pollution. (IQAir)

Why does it often improve later in the day?

As the sun warms the ground:

• the inversion weakens,
• the air begins to mix,
• the wind often picks up,

so the pollution disperses and the AQI frequently drops by late morning or the afternoon.

That’s one of the reasons why, when I checked our hike conditions at 07:30 this past Saturday the AQI was 55 (Moderate). If I had checked again around midday, it may well have been lower.

What does “Unhealthy for Sensitive Groups” mean?

It doesn’t necessarily mean everyone will feel unwell.

The people most likely to be affected are:

• children,
• adults over 65,
• people with asthma,
• people with COPD or other lung diseases,
• people with heart disease.

Healthy adults can usually continue normal activities, but it’s sensible to reduce prolonged, very strenuous outdoor exercise if the AQI is in that range. (aqicn.org)

For our hiking group

It will be interesting to record AQI before and after each hike. It might also be useful to include a simple interpretation, for example:

AQI	Meaning	Walking recommendation
0–50	Good	Excellent conditions
51–100	Moderate	Suitable for most people
101–150	Unhealthy for Sensitive Groups	Sensitive individuals should take extra care
151+	Unhealthy	Consider shortening or postponing strenuous activities

That makes the numbers much more meaningful to everyone.

One interesting thing to watch

The hourly AQI graph in IQAir before and after our walks/ hikes/runs could show a pattern as follows:

• 06:00–08:00 → AQI highest
• 09:00–11:00 → gradually improving
• 12:00–15:00 → often the cleanest air of the day
• Evening → pollution begins to rise again as temperatures drop and more heating fires are lit

It will be useful adding environmental information to our hiking posts. It fits perfectly with the theme of healthy outdoor living.

[Author]

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