The sky turns hazy, the air smells faintly of smoke, and suddenly everyone is checking a number on their phone. But what do PSI and API actually measure, and what should you do differently when they climb?
Key Takeaways
- PSI (Pollutant Standards Index) is used in Singapore, while API (Air Pollutant Index) is used in Malaysia. Both track similar pollutants on a comparable scale.
- Readings above 100 are generally considered unhealthy, particularly for sensitive groups.
- Children, the elderly, and those with respiratory or heart conditions face higher risk during haze.
- Simple steps, such as staying indoors and using a well-fitted mask outdoors, can meaningfully reduce exposure.
What Do PSI and API Actually Measure?
Both indices summarise several air pollutants, including fine particulate matter (PM2.5 and PM10), ozone, carbon monoxide, sulphur dioxide, and nitrogen dioxide, into a single number. During haze episodes, PM2.5 is usually the main driver, since these tiny particles can travel deep into the lungs and even enter the bloodstream, an effect the World Health Organization highlights as a leading contributor to air pollution-related illness.
The two indices are broadly comparable, both running on a scale where higher numbers mean worse air quality:
- 0 to 50: Good
- 51 to 100: Moderate
- 101 to 200: Unhealthy
- 201 to 300: Very Unhealthy
- Above 300: Hazardous
Singapore’s National Environment Agency publishes PSI readings and guidance for the public, while Malaysia’s Department of Environment maintains the equivalent API system. A reading in the “unhealthy” range does not mean everyone will feel unwell immediately, but it does mean prolonged or strenuous outdoor activity becomes riskier, especially for sensitive groups.
Who Should Be Most Careful?
Children’s lungs are still developing, and they tend to breathe faster relative to their size, so they inhale proportionally more pollutants during outdoor activity. Older adults, pregnant women, and people with asthma, chronic lung disease, or heart conditions are also more vulnerable to haze-related symptoms such as coughing, breathlessness, eye irritation, and fatigue.
What Should Families Actually Do?
Rather than fixating on a single number, it helps to think in terms of practical steps:
- Check readings regularly from official sources, such as Singapore’s haze microsite, rather than relying on how the sky looks.
- Reduce prolonged outdoor exertion when readings enter the unhealthy range, particularly for children and elderly family members.
- Keep windows closed and use an air purifier indoors if pollution is persistent.
- Use a properly fitted N95 or equivalent mask outdoors when readings are high, noting that cloth or surgical masks offer far less protection against fine particles.
- Watch for symptoms such as persistent coughing, wheezing, or chest discomfort, and seek medical advice if they occur or worsen.
The Bottom Line
Haze season brings a familiar routine of checking numbers, but those numbers exist to guide real decisions, not just anxiety. Understanding what PSI and API represent and adjusting outdoor activity accordingly is a more useful response than guessing based on how hazy the air looks.
Editorial Note
This article is intended for general information and should not replace personalised medical advice. Individual sensitivity to haze varies, and families with existing respiratory or cardiovascular conditions should follow guidance from their healthcare provider during prolonged haze episodes.
Sources
- National Environment Agency Singapore. “Haze Situation Update (24 September 2026).” National Environment Agency Singapore, 2026. https://www.nea.gov.sg/media/news/advisories/index/haze-situation-update-24-september-2026
- National Environment Agency Singapore. Haze Situation Update”, National Environment Agency Singapore, 2026. https://www.haze.gov.sg
- Department of Environment Malaysia. “Air Pollutant Index Management System (APIMS).” Department of Environment Malaysia, 2026. https://eqms.doe.gov.my/APIMS/main
- World Health Organization. “Ambient (Outdoor) Air Quality and Health.” World Health Organization, 2024. https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health