Air Quality Index Calculator

Understanding the Air Quality Index Calculator

An air quality index lookup converts an AQI number into a category and health advice. The scale is designed so that 100 marks the level at which a pollutant reaches its short-term health standard, which is why the advice changes character rather than degree at that point.

How it actually works

Enter an AQI reading. The tool matches it to one of the standard bands and returns the corresponding category and guidance. A reading of 80 falls into Moderate, where air quality is acceptable but unusually sensitive people should limit prolonged outdoor exertion.

The deeper context most people miss

The index reports the single worst pollutant rather than a combined measure, so an AQI of 80 could reflect ozone, fine particulate matter, or something else entirely. Two locations with the same number can therefore call for different responses, since ozone peaks in afternoon sun while particulate pollution behaves quite differently.

Why fine particulate matter matters most

The pollutant driving AQI on most bad air days is PM2.5, particulate matter smaller than 2.5 micrometres, and its size is what makes it significant. Particles that small bypass the upper airway defences that trap larger dust, reach deep into the alveoli, and some fraction crosses into the bloodstream. The health evidence is substantial and covers more than lungs: long-term exposure is associated with cardiovascular disease, stroke, reduced lung development in children, and increased all-cause mortality, and short-term spikes are associated with hospital admissions for respiratory and cardiac events within days. The World Health Organization revised its air quality guidelines in 2021, lowering the recommended annual PM2.5 limit substantially on the basis of accumulated evidence, and the revised guideline sits well below what many national standards permit and below what a large majority of the world's population actually breathes. Importantly, the evidence has not identified a threshold below which PM2.5 is harmless, which is why guidance emphasises reduction rather than compliance with a bright line. Ozone, the other common driver, forms photochemically when sunlight acts on precursor pollutants, which is why ozone AQI peaks on hot sunny afternoons and falls overnight, an entirely different daily pattern from particulate pollution driven by wildfire smoke, traffic, or heating.

A worked example: what to actually do at a given reading

At an AQI of 80, most people need change nothing, and someone with asthma may notice a difference during hard exercise. At 150, the practical responses are concrete: reduce time outdoors, move strenuous activity indoors or reschedule it, close windows if outdoor air is worse than indoor, and run air filtration if available. Above 200, those responses apply to everyone rather than to sensitive groups. The specific action that matters most during particulate events is filtration, since a HEPA filter removes fine particles effectively and a well-sealed room with a suitably sized purifier can reduce indoor PM2.5 substantially below outdoor levels. Purifier sizing matters and is expressed as clean air delivery rate, which should be matched to room volume with several air changes per hour, and an undersized unit in a large room achieves little. A widely used low-cost alternative, sometimes called a Corsi-Rosenthal box, tapes furnace filters to a box fan and performs surprisingly well for the cost, which matters where commercial purifiers are unaffordable or unavailable during a smoke event. Masks help outdoors, and the distinction matters: cloth and surgical masks do little for fine particulate, while a properly fitted N95 or FFP2 respirator filters it effectively, with fit being the determining variable rather than the rating alone.

Deciding how to interpret readings and forecasts

Several practical points affect how a number should be read. AQI is calculated from a rolling average rather than an instantaneous measurement, with PM2.5 typically using a 24-hour or shortened rolling window depending on the agency, which means a reading lags a rapidly changing situation such as approaching wildfire smoke. Checking the underlying pollutant concentration alongside the index, which most monitoring sites publish, gives a more current picture. Monitor location matters, since official monitors are sparse and a reading from several miles away may not represent your street, particularly for traffic-related pollution which varies sharply over short distances. Low-cost sensor networks have proliferated and provide much denser coverage, with the caveat that consumer sensors are less accurate than reference monitors, tend to overread in high humidity, and benefit from correction algorithms that some networks apply. Different countries use different index scales, and this is a genuine source of confusion: the US, European, Chinese, Indian, and UK indices use different breakpoints and even different ranges, so an AQI of 100 does not mean the same thing everywhere and comparing a reading against advice written for another country's scale can mislead substantially in either direction.

Indoor air, which is where most exposure happens

People in developed countries spend the large majority of their time indoors, so indoor concentrations dominate total exposure regardless of outdoor readings. Indoor air draws from outdoor air plus indoor sources, and the indoor sources are frequently the larger problem. Cooking is a major one, particularly gas cooking, which produces nitrogen dioxide alongside particulates, and frying and high-heat cooking generate substantial PM2.5 regardless of fuel; extraction ventilation venting outdoors, used consistently, is the effective control and many kitchen hoods recirculate rather than vent, which does little for gases. Candles, incense, and wood burning stoves are significant particulate sources. Smoking indoors dominates everything else. Volatile organic compounds come from paints, adhesives, cleaning products, and new furnishings. Radon is a separate and serious concern in some geologies, being a leading cause of lung cancer after smoking, and it is detectable only by testing. Carbon dioxide accumulates in poorly ventilated occupied rooms and, while not directly hazardous at typical indoor levels, serves as a useful proxy for ventilation adequacy, with readings above roughly 1,000 parts per million indicating that fresh air exchange is inadequate. Ventilation and source control are the two levers, and filtration addresses particles while doing nothing for gases.

Variations: national indices, pollutants, and sensitive groups

The US EPA index runs 0 to 500 with six categories and is calculated for ozone, PM2.5, PM10, carbon monoxide, sulphur dioxide, and nitrogen dioxide, reporting the highest. The UK Daily Air Quality Index runs 1 to 10 in four bands. The European Air Quality Index uses six categories with its own thresholds. China and India operate their own. Because breakpoints differ, converting between them is not straightforward and reading advice intended for a different scale is a real hazard. On pollutants, PM2.5 and ozone dominate most exceedances, PM10 matters where dust is significant, nitrogen dioxide is largely traffic-related and varies sharply near busy roads, and sulphur dioxide has declined substantially in many countries with coal reduction. Sensitive groups conventionally include people with asthma, COPD, and other lung conditions, people with cardiovascular disease, children whose lungs are still developing and who breathe more air per unit body weight, older adults, pregnant women, and outdoor workers who cannot easily reduce exposure. For anyone in these groups, having a personal action plan agreed with a clinician, including when to adjust medication during poor air quality episodes, is more useful than generic advice.

Using air quality readings well

Check which pollutant is driving the index, since ozone peaks on hot sunny afternoons while particulate pollution follows entirely different patterns and calls for different timing of outdoor activity. Note which country's scale a reading uses, since breakpoints differ substantially and advice written for one index misleads when applied to another. Look at the underlying concentration alongside the index during rapidly changing events, since AQI uses rolling averages and lags approaching smoke. Prioritise filtration during particulate episodes, sizing a purifier to the room by clean air delivery rate, and consider a box fan filter build where cost is a barrier. Use a properly fitted N95 or FFP2 outdoors rather than a cloth or surgical mask, since fit determines effectiveness for fine particles. Attend to indoor sources, particularly cooking with extraction that actually vents outdoors, since indoor concentrations dominate total exposure. And if you have asthma, heart or lung disease, agree an action plan with a clinician rather than relying on generic category advice.

What people get wrong

• Comparing an AQI reading against advice written for a different country's index, when breakpoints and ranges differ substantially between the US, UK, EU, Chinese, and Indian scales.
• Treating the index as a combined measure, when it reports only the single worst pollutant and two identical readings can call for different responses.
• Relying on a cloth or surgical mask during smoke events, when neither filters fine particulate effectively and a fitted N95 or FFP2 is required.
• Focusing on outdoor readings while ignoring indoor sources, when most exposure occurs indoors and cooking, candles, and wood burning frequently dominate.

Where the math comes from

The index maps a measured pollutant concentration onto a 0 to 500 scale using breakpoints set so that 100 corresponds to the short-term health standard for that pollutant. Where several pollutants are measured, the reported AQI is the highest individual value rather than a combination. Concentrations are averaged over defined periods, so readings lag rapidly changing conditions.

Questions and answers

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