Delhi is once again battling worsening air pollution as the onset of winter brings a familiar deterioration in air quality to India’s capital.

The city’s air quality index (AQI) – which measures PM2.5, fine particulate matter that can clog lungs – has crossed 300, entering the “very poor” category and raising health concerns for millions of residents.

Breathing such polluted air over prolonged periods can cause respiratory problems and worsen existing illnesses, with children and older people particularly vulnerable.

Authorities have responded by invoking Stage II of the Graded Response Action Plan (GRAP), introducing measures to curb dust, vehicle emissions and other sources of pollution.

GRAP is a set of emergency measures that become progressively stricter as pollution levels rise. The latest restrictions include more intensive road cleaning, higher parking charges to discourage private vehicle use and increased public transport services. Delhi Metro has introduced 20 additional weekday train trips, while authorities have been directed to stagger government office timings to ease traffic congestion.

Officials said the measures were introduced after forecasts suggested air quality would remain very poor over the coming days.

Delhi and neighbouring parts of northern India grapple with hazardous air every winter, particularly around Diwali, one of India’s biggest festivals, which is just days away.

The festival is celebrated with lights, lamps and fireworks, which often worsen the city’s already poor air quality by releasing fine particles and harmful gases into the atmosphere.

The annual pollution crisis is driven by a combination of factors, including vehicle exhaust, industrial emissions, falling temperatures, low wind speeds and the burning of crop stubble in neighbouring states.

The sharp rise in pollution follows an unusually clean October, when Delhi recorded its lowest average AQI for the month in eight years.

Experts have attributed the sudden deterioration to falling temperatures, slow winds and an increase in crop-residue burning in the neighbouring states of Punjab and Haryana.

A recent storm had brought rain and stronger winds, helping clear the air. But as the weather settled, northwesterly winds returned, carrying smoke from burning fields towards the capital.

India’s pollution watchdog classifies AQI readings between 301 and 400 as “very poor”, while anything above 400 is considered “severe”. On Wednesday morning, Delhi’s AQI stood at 377.

The index takes into account eight major pollutants, including PM2.5 – tiny particles about 30 times smaller than the width of a human hair that can penetrate deep into the lungs.