NEW DELHI: If breathing the city’s toxic air is akin to smoking 25–30 cigarettes a day, a morning walk in the haze could make you feel like a chronic smoker even if you haven’t touched a cigarette in your lifetime.With the air quality slipping into ‘very poor’ category in general and ‘severe’ in some areas, doctors warn that outdoor exercise in the early hours may do you more harm than good.

During high-pollution days, especially when the Air Quality Index (AQI) crosses 300, outdoor physical activity exposes one to high levels of fine particulate matter (PM2.5 and PM10). These microscopic pollutants penetrate deep into the lungs and can aggravate respiratory and heart conditions.

“On such days, walking and jogging in the morning expose your lungs to these pollutants,” said Dr Neeraj Nischal from the department of medicine at AIIMS. “During physical activity, breathing becomes deeper and faster, pulling the particles further into the lungs. This can irritate airways, reduce oxygen delivery, and worsen heart and lung health.”Pollution tends to peak early in the morning and late at night because of temperature inversion — when cooler air near the ground traps pollutants. “Air quality typically improves slightly by late morning or afternoon as sunlight disperses pollutants,” he said. Dr Pulin Gupta of the department of medicine at RML Hospital echoes Nischal’s views. “In early winter, when temperatures drop and wind speed is low, morning walkers are at risk. exposed to a higher level of pollutants. So, it’s better to walk in the afternoon rather than early morning or late evening.”Wearing N95 or surgical masks can reduce this exposure but may restrict breathing during workout. “The body needs more oxygen while brisk walking and jogging, and restricted airflow can cause dizziness resulting in injuries due to a fall. Gentle walking is safer, but running with a tight mask should be avoided,” Gupta said. Dr Arunesh Kumar, director & head of pulmonology, Paras Health, Gurgaon, agreed that while a properly fitted N95 or KN95 mask can offer partial protection, it should not be worn during exercise. “Light walking is acceptable during high-pollution spells, but strenuous workouts like running or cycling should be altogether avoided,” he said.“Even short-term exposure to foul air can cause cough, sore throat, tightness in chest and breathlessness,” said Dr Rakesh Pandit, senior consultant & head of internal medicine at Aakash Healthcare. “People with asthma, COPD and heart ailments may see their symptoms worsen. Long-term exposure increases the risk of chronic lung and heart diseases and even stroke.”Dr Neetu Jain of PSRI Hospital advises that when the AQI crosses 200, it’s best to avoid all outdoor physical activity. “Children, the elderly and those with chronic illnesses should stay indoors even when the index is above 150. Workouts, such as yoga or treadmill sessions are safer alternatives,” she said.Recent research reflects these medical concerns. A 2023 study in The Lancet Planetary Health found that while exercise normally boosts our cardiac health, this benefit decreases, or may even reverse, when done in polluted air. The study, which tracked over 3 lakh adults in China, showed that people exposed to PM2.5 levels above 54 µg/m³ did not gain the same cardiovascular benefits from walking or cycling as those in cleaner environment. A related commentary in the journal consequently described such exercises as a “two-edged sword”.As Delhi’s air grows hazier with the onset of winter, doctors are unanimous: skip the morning and evening walk and move your workout indoors.