The distinction between the Pollutant Standards Index (PSI) and the Air Quality Index (AQI) has become particularly relevant as air quality concerns rise globally. These indices provide different scores for air pollution levels, which can lead to confusion when monitoring air quality in the same location. Both indices aim to inform the public about the safety of the air they breathe, but they do so using different methodologies and pollutant considerations.

The AQI is a widely recognised measure that indicates how polluted the air is and what health implications may arise from exposure. It evaluates various pollutants, including PM2.5, PM10, ozone, and nitrogen dioxide. In contrast, the PSI serves a similar purpose but utilises region-specific calculations to convey air quality and associated health risks, as explained by Dr Samir Garde, Director of the Department of Pulmonology and Lung Transplant at Gleneagles Hospital in Parel.

Historically, the PSI was the original term used in the United States during the 1970s before it was renamed the AQI. Singapore adopted a similar index but retained the PSI nomenclature, making it the official air quality measure in that region.

Reasons for Discrepancies in Readings

Disparities between PSI and AQI readings can arise due to various factors, particularly highlighted during recent haze episodes. For instance, on 10 October, Singapore’s National Environment Agency (NEA) reported a PSI of 127-177, categorising the air quality as “unhealthy.” Simultaneously, IQAir’s platform indicated a US AQI of 197, also classified as “unhealthy” but at the upper end of the scale.

Dr Garde notes that differences in readings can stem from variations in the pollutants measured, the calculation methods employed, the averaging periods used, and the rating scales applied. IQAir identifies two primary reasons for these discrepancies: timing and strictness. The PSI is updated hourly but reflects an average of the previous 24 hours, which may not accurately represent real-time conditions. Conversely, IQAir’s data is reported in near real-time, allowing for immediate visibility of sudden changes in air quality.

Moreover, the US AQI is considered one of the more stringent indices globally. For example, while Singapore’s PSI categorises PM2.5 levels between 13 and 55 micrograms per cubic metre as “moderate,” the US AQI considers a narrower range of 35.5 to 55.4 g/m as “unhealthy for sensitive groups.” This difference in classification further contributes to the variation in reported scores.

Health Implications of Air Quality Scores

Higher readings on either the PSI or AQI indicate increased air pollution and a greater risk of health issues. Dr Garde emphasises that elevated scores correlate with a higher likelihood of respiratory problems, throat irritation, and other related symptoms. Vulnerable populations, including children, older adults, and individuals with pre-existing health conditions, face heightened risks during periods of poor air quality.

It is crucial to recognise that a seemingly good air quality score may not reflect the full spectrum of pollutants present or account for local pollution hotspots. Short-term spikes in pollution levels or indoor pollution can be overlooked in averages. Therefore, Dr Garde advises monitoring individual pollutant levels and local advisories for a comprehensive understanding of air quality.

When assessing air quality, it is advisable to rely on the official index provided by local environmental authorities. In Singapore, this is the PSI, while in India, the national AQI serves this purpose. For comparisons across different countries, examining specific pollutant concentrations, such as PM2.5, is recommended to gain a clearer perspective.

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