New Delhi- Schools were shut across India’s capital on Friday as a noxious grey smog engulfed the megacity and made life a misery for its 30 million inhabitants. Smoke from farmers burning crop stubble, vehicle exhaust and factory emissions combine every winter to blanket Delhi in a choking haze. The public health crisis has persisted for decades and researchers have blamed the smog for hundreds of thousands of premature deaths across India. Levels of the most dangerous PM2.5 particles -- so tiny they can enter the bloodstream -- were on Friday almost 35 times the daily maximum recommended by the World Health Organization, according to monitoring firm IQAir. “In light of the rising pollution levels, all govt and private primary schools in Delhi will remain closed for the next 2 days,” chief minister Arvind Kejriwal wrote on X, formerly Twitter. Delhi, one of the largest urban areas on the planet, is also regularly ranked as one of the world’s most polluted cities. Visible smog is a burden for residents through much of the year but the problem peaks at the start of winter around the Hindu festival of Diwali. The holy day coincides with the weeks when tens of thousands of farmers across north India set fire to their fields to clear crop stubble from recently harvested rice paddies. That practice is one of the key drivers of Delhi’s annual smog problem, worsening the impact of vehicle and industrial emissions.