When AQI crosses 400: How toxic air damages your lungs, heart, and brain

An AQI above 400 turns the air into a slow poison. Doctors explain how invisible pollutants invade your bloodstream, trigger inflammation, and put your heart, lungs, and brain at risk

Delhi woke up to yet another morning shrouded in smog. The city’s average Air Quality Index (AQI) stood at 425 on Tuesday, in the “severe” range. Several pockets such as Anand Vihar, Wazirpur, and Mundka crossed 440.

 

An average adult breathes roughly 20,000–22,000 times a day, more when active, and not every one of those breaths can be behind a mask. As the capital battles this toxic mix, doctors warn what severely polluted air actually does inside your body.

 

When AQI goes above 400, every inhalation becomes a delivery vehicle for microscopic pollutants such as particulate matter 2.5 (PM2.5), PM10, nitrogen dioxide, sulphur dioxide, and even heavy metals.

 

“These particles are smaller than a strand of hair,” explains Dr Ankit Kumar Sinha, Senior Consultant and Unit Head, Pulmonology, Respiratory & Sleep Medicine at Max Super Speciality Hospital, Vaishali. “They slip past your body’s natural defences, travel deep into the lungs, and reach the tiny air sacs where oxygen is exchanged.”

 

Within hours, your airways swell, and the body launches an immune reaction leading to inflammation. However, this is not the helpful kind of inflammation you get when fighting an infection. “It’s a constant, uncontrolled immune response that starts harming your own tissues,” says Dr Sinha.

 

This results in burning eyes, coughing, breathlessness, and fatigue.  

Does this only affect your lungs? 

“Absolutely not,” says Dr Naresh Trehan, Chairman and Managing Director, Medanta – The Medicity, in a conversation with reporters. “These particles don’t stay in the lungs. They get absorbed into the bloodstream and travel to the kidneys, liver, heart, and even the brain.” 

 

He warns that air pollution spikes have been linked to higher blood pressure, arrhythmias, strokes, and heart attacks. “It’s a disaster for public health,” says Dr Trehan. “People already living with heart, liver, or kidney conditions find their disease worsening rapidly during these pollution surges.”

 

Repeated exposure can also age the body faster. This phenomenon, called biological ageing, occurs when constant oxidative stress and inflammation slow down your body’s repair mechanisms, not just in the lungs, but across every organ system.  

Why are children and pregnant women most at risk? 

Children are among the worst affected. Their lungs are still developing, and they breathe faster than adults, meaning they take in more air, and more toxins, per minute.

 

“In kids, pollutants can slow lung growth and cause lifelong breathing problems,” warns Dr Sinha. “For pregnant women, exposure to such high pollution levels can lead to low birth weight or even premature delivery.”

 

Dr Trehan adds that there is growing evidence that pollution also impacts a child’s neurological development. “We’re seeing long-term effects on the brain, reduced cognitive function, learning difficulties, and delayed milestones.” 

 

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This content is for informational purposes only and is not a substitute for professional medical advice.