On a crisp winter morning, as the sun struggles to break through a gray, smoky haze, many of us glance at the air-quality index like a daily horoscope, the number that decides whether we’ll wear a mask, skip a jog, or brace for another day of breathing toxic air. But what do these figures actually mean for our lungs? Why does 200 feel unbearable and 500 feel apocalyptic? And as the smoke from stubble burning mixes with vehicles and industrial smoke pollutionWhat are all those particles actually doing inside the body?Here’s a look at how AQI works and what a pulmonologist wants you to know before the next toxic plume hits.
Why AQI is not just a number: Decoding what it measures
For years, India’s air-quality data was hidden in technical charts and numbers, leaving most people unsure of what the readings meant for the air they breathed. Without a clear, accessible system, public engagement with pollution remained limited.To change this, the Central Pollution Control Board (CPCB) planned to create a national air quality index (AQI) that could translate complex measurements into simple, accessible categories. An expert group bringing together doctors, air-quality scientists, academics, NGOs, state pollution boards and technical inputs from IIT Kanpur ultimately developed India’s AQI. A draft was released in October 2014 for public and institutional feedback and after detailed review, the national framework was finalized.AQI is designed to tell people at a glance not only how polluted the air is, but what pollution levels could mean for their health. It classifies air quality into six categories: good, satisfactory, moderately pollutedbad, very bad and seriousThe index accounts for eight major pollutants with 24-hour standards and calculates separate sub-indices for each, so as long as there is data for at least three pollutants, necessarily one of them PM2,5 or PM10, an AQI can be calculated, The highest (or “worst”) sub-index becomes the overall AQI number for that location.AQI measures major pollutants such as PM2.5, PM10, ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide. Among them, PM2.5 is considered the most harmful as its tiny particles can penetrate deep into the lungs and even enter the bloodstream, causing serious health risks. World Health Organization (WHO) recommends keeping PM2.5 levels below 5 micrograms per cubic meter (µg/m³) annually and below 15 µg/m³ for 24 hours.
Why is there 500 in India and not 500 in America?
America’s AQI, like India’s, runs on a 0-500 scale, but with one important difference. Its highest category, “Hazardous”, starts at 301 with no upper limit, so readings can go well beyond 500. The two countries also use different formulas to calculate their scores, meaning the same pollution data can yield different AQI values. In short, the Indian and American scales are not exactly the same and cannot be compared directly.
How stubble fires promote winter haze
Every winter, the vast expanse of northern India from Punjab to Haryana In neighboring states, farmers can be seen burning crop residue as they clear their fields for the next sowing season. This practice, known as crop residue burning (CRB), remains a persistent environmental challenge in many agricultural regions in India and around the world.CRB is a major driver of fine particulate pollution, especially PM2.5, that engulfs Delhi and the wider NCR region every winter. These microscopic particles pose a health threat not only to residents but also to farmers living near the burnt sites. Because PM2.5 particles are small enough to penetrate deep into the respiratory system, they can lodge along the delicate lining of the alveoli in the lungs.
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A growing body of scientific evidence links PM2.5 exposure to significant health risks. Short-term exposure can impair lung function and make conditions such as asthma and heart disease worse. In the long term, the threats intensify; Long-term inhalation has been shown to cause chronic bronchitis, reduced lung capacity, and increased mortality from lung cancer and cardiovascular disease.
Eleven cities, zero clean air days: Delhi is the worst of them all
Delhi once again tops the charts as India’s most polluted city, according to a decade-long AQI assessment by Climate Trends covering 2015 to November 2025. The analysis shows that no major Indian city has reached safe air-quality levels at any time in the last ten years, underscoring the severity of the national pollution crisis.
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Delhi was the most affected, followed by Noida, where a dangerous AQI of 515 was recorded. Other highly polluted areas include Cooch Behar (362), Kishanganj (351), and Kultali (332), showing how the toxic air has spread beyond the metros and into smaller districts. The data points to a nationwide public-health emergency, with the capital remaining at the epicenter of India’s pollution burden.
‘Acute respiratory distress’: How AQI 200 vs 500 affects your body
Dr. Kuldeep Kumar Grover, head of critical care and pulmonology at CK Birla Hospital, Gurugram, says that the body reacts almost immediately when pollution levels increase. Fine particles cause irritation and inflammation in the airways, leading to cough, excess mucus and reduced lung function, while the heart and blood vessels are put under additional stress due to oxidative stress, narrowed vessels and increased tendency to clot.“The cardiovascular system also responds to pollution by inducing oxidative stress, blood vessel contraction and an increased tendency for blood clotting.”
Citizens protest against pollution in Mumbai (PTI)
At an AQI of around 200, the body experiences noticeable but manageable stress; Symptoms such as throat irritation, headache and breathlessness are common, especially for vulnerable groups. But when the level reaches 500, the risk becomes excessive. Inflammation intensifies, asthma can rapidly worsen, blood pressure can increase, and even healthy people may struggle with oxygen exchange. Dr. Grover says continued exposure to this severity can lead to acute respiratory or cardiovascular complications.
PM2.5: Micropollutants with massive health consequences
Surface pollution from stubble burning and urban sources exposes people to major winter pollutants PM2.5, PM10 and carbon monoxide. The smallest of these, PM2.5, can overcome the body’s natural defenses, penetrating deep into the lungs and even entering the bloodstream. Carrying toxic metals and reactive chemicals, these particles increase chronic inflammation and gradual tissue damage. Over time, doctors see a rapid decline in lung function, frequent bronchitis, worsening asthma, and a higher risk of heart and vascular disease.Dr Grover said, “Long-term exposure also increases the risk of lung cancer, premature birth and impaired cognitive development in children. The cumulative effect of this pollution is similar to the long-term effects of smoking in some highly polluted areas.”
When pollution increases, who is most vulnerable?
Children, pregnant women and older adults face greater risk as AQI levels rise because their bodies are more sensitive to pollution-induced stress. Children breathe faster and their lungs are developing, making them more likely to cough, wheeze and fatigue if AQI levels are above 150.Pregnant women, who already have low oxygen stores, may experience difficulty breathing when pollution rises above 200, and prolonged exposure has been linked to complications such as “low birth weight and premature delivery”. Older adults, especially those with heart or lung conditions, may develop chest tightness, irregular heartbeats or worsening COPD even at an AQI 150-200, Dr. Grover said.
India Gate covered in smog (ANI)
On the other hand, for people suffering from asthma or chronic bronchitis, polluted air aggravates existing inflammation. Irritants such as PM2.5 and ozone narrow airways, increase mucus and increase sensitivity to triggers, leading to frequent wheezing, coughing, breathlessness and greater reliance on rescue inhalers. High pollution days also increase the risk of asthma attacks, respiratory infections and rapid decline in lung function.“Patients should follow their treatment plans closely, avoid outdoor activities during times of peak pollution, and use preventive medication as recommended by their physician,” Dr. Grover said.
How to protect yourself in bad days?
Dr Grover said that on days when air quality drops into the “very poor” or “severe” category, the most important strategy is to reduce overall exposure.Indoors, pollutant levels can be significantly reduced by keeping windows closed, avoiding smoke or sunlight, and using a certified HEPA purifier. Outdoors, it’s best to limit activity, especially exercise, which draws particles deeper into the lungs. Only properly fitted N95 or FFP2 masks provide real protection; Clothing and surgical masks do little to prevent PM2.5. Supportive measures such as staying hydrated, using nasal saline rinses, and monitoring symptoms closely help reduce irritation. People with chronic lung or heart disease should keep medications with them and follow their action plans carefully. Avoiding peak hours of pollution, typically morning and evening, is one of the simplest ways to reduce exposure.As winter smog thickens and AQI levels rise from unhealthy to severe, the message from experts is clear: Pollution is not just an environmental issue, but a direct, measurable threat to health.From choosing the right mask to avoiding peak smog hours and protecting vulnerable groups, even small, consistent steps can significantly cut risk. And while individual actions provide only partial protection, experts say awareness is the first line of defense until long-term, systemic solutions begin to clean the air once and for all.
