India’s Supreme Court has called on New Delhi to clarify how it measures air quality in one of the world’s most polluted cities, as critics claim authorities spray water near monitoring stations and cap measurements to influence the readings.

From late October New Delhi is blanketed in a toxic haze for several months, with fireworks from Diwali, the festival of lights, mixing with smoke from farm fires, fumes from cars, industrial emissions, construction sites and power plants to muddy the air.

In its latest request for evidence in a four-decade long case on the impact of pollution brought by activists who say it is harming their health, the Supreme Court asked the city government to clarify what devices it used to measure the air and how they worked.