Winter is coming, and Delhi is preparing for another season with lung-choking smog.

Some eight residents of India’s capital are thought to die each day because of the city’s notoriously polluted air. Though authorities have been trying to clean it up by cutting traffic and the use of coal in power plants, the problem hasn’t really gone away. The newest culprit seems to be what has become coal’s replacement: an even dirtier fuel called petroleum coke.

Petcoke, as it is commonly called, starts its life in the pits of Canada’s tar sands, which are one of the dirtiest sources of crude oil. It is then refined in the US Gulf Coast, where the lighter and most valuable fractions, such as petrol and diesel, are removed. What is left behind is petcoke, which cannot be burned in the US and most other countries because it produces more carbon, sulfur, and heavy-metal emissions than even coal. To try and make some money on what would otherwise be waste, US companies export petcoke to markets that still use them, including India and China.

Since 2014, China has been cutting its use of petcoke. That leaves India as the biggest importer of the dirty fuel.

Even India’s use of petcoke is coming under fire. In February, the country’s supreme court ordered the government to ban the use of the fuel in power generation or limit the amount of sulfur emitted in the process. For comparison, India’s lax regulations say that the amount of sulfur contained in fuels should be no more than 4,000 parts per million (ppm) but the regional environmental agency says that petcoke contains 72,000 ppm of sulfur.

However, the government has failed to ban or regulate the fuel. So the court has come up with a deadline of Oct. 24 for the government to come up with a plan. The deadline is after India’s largest festival, Diwali, when people burst lots of firecrackers. Anticipating the problem, the country’s supreme court has banned the sale of firecrackers until after Diwali is over. Be assured, however, that simply not bursting firecrackers isn’t going to make Delhi’s smog problem go away.