What the AQI Actually Measures
Launched in 2014, India’s National Air Quality Index was designed to simplify complex data into a single, understandable figure. It condenses the concentration of eight major pollutants into one number, colour, and description, ranging from 'Good' to 'Severe'.
The pollutants tracked are Particulate Matter (PM10 and PM2.5), Nitrogen Dioxide (NO2), Sulphur Dioxide (SO2), Carbon Monoxide (CO), Ozone (O3), Ammonia (NH3), and Lead (Pb). The genius of the AQI is its simplicity. For a valid AQI to be calculated, data for at least three pollutants must be available, one of which must be either PM10 or PM2.5. The final number you see reflects the pollutant with the highest health risk at that moment. So, if a city's AQI is 'Poor' because of high ozone levels, it will be labelled 'Poor' even if the other seven pollutants are in the 'Good' range.
The Danger in the Details: PM2.5 vs. PM10
The AQI reports on both PM10 and PM2.5, but understanding the difference is crucial. Both are inhalable particles, but their size determines how dangerous they are. PM10 refers to coarse particles with a diameter of 10 micrometres or less, such as dust from construction sites or roads. These are generally trapped in the upper airways like the nose and throat. PM2.5, or fine particles, are 2.5 micrometres or smaller—so tiny they can bypass the body's natural defences, penetrate deep into the lungs, and even enter the bloodstream. These particles are often from combustion sources like vehicle exhaust, industrial emissions, and burning wood or waste. Long-term exposure to PM2.5 is linked to serious health issues, including premature death, chronic heart and lung diseases, and reduced lung function in children. While the AQI includes PM2.5, the single number doesn't always convey the specific risk posed by these more dangerous fine particles compared to their coarser counterparts.
The Pollutants Left Behind
While India's AQI is relatively comprehensive, it doesn't track everything. Notably absent are pollutants like carbon dioxide (CO2) and methane (CH4). These are primarily categorized as greenhouse gases that drive climate change and do not have the same immediate, acute health effects at typical atmospheric levels as the eight criteria pollutants. Also missing are Volatile Organic Compounds (VOCs), which are gases emitted from paints, cleaning supplies, and furniture. These can cause a range of health problems and contribute to the formation of ground-level ozone, a major component of smog. The AQI’s focus is on short-term health impacts from a specific list of pollutants, meaning other potentially harmful substances in the air are not reflected in the daily score.
The Indoor Air Quality Blind Spot
Perhaps the biggest limitation of the AQI is that it only measures outdoor, or ambient, air quality. Considering that people spend up to 90% of their time indoors, this is a significant gap. Shockingly, indoor air can be two to five times more polluted than the air outside. Outdoor pollutants like PM2.5 easily seep indoors through windows and ventilation systems. Furthermore, indoor environments have their own sources of pollution, including cooking fumes, cleaning agents, mould, and VOCs from furnishings. Energy-efficient, well-sealed buildings can sometimes exacerbate the problem by trapping these pollutants inside. This can lead to what is known as 'sick building syndrome,' with symptoms like headaches, fatigue, and difficulty concentrating. Your home or office could have poor air quality even on a day when the outdoor AQI is 'Good'.
From Numbers to a Holistic View
The AQI is a valuable starting point, not the final word. To get a more complete picture of your air quality exposure, it's important to look beyond the single number. Many air quality apps and websites, like SAFAR and CPCB's portal, provide a breakdown of the individual pollutants that make up the AQI. Paying attention to the levels of PM2.5 and ozone, which often pose the greatest health risks, can offer more insight than the composite score alone. Furthermore, be mindful of your indoor environment. Investing in air purifiers with HEPA filters, ensuring good ventilation by opening windows when outdoor air is cleaner, and choosing low-VOC products can significantly reduce indoor pollution. The goal is to move from passively receiving a daily score to actively managing your personal exposure, both inside and out.
