A Greenpeace India report released on Thursday shows that samples collected from classrooms in Delhi between October and November show high levels of heavy metals during days when particulate matter 2.5 readings were high. Greenpeace noted the need for action to reduce air pollution in northern India and suggested precautionary measures, including shutting schools.

The report said that monitors installed inside the classrooms showed poor air quality levels, five times above Indian safety standards, and 11 times above World Health Organisation standards. Carcinogenic heavy metals including arsenic, cadmium, lead, and nickel were detected.

The Delhi Pollution Control Committee said the report could be based on skewed readings, as WHO standards and Indian air quality standards are defined by annual averages, while the report was based on readings from single spots across one day only.

Experts cautioned that the trend is not limited to Delhi alone, with large parts of North India facing severe air pollution problems. This is exacerbated by the industrial belts that dot the area.