(DNAINFO)  Emily Frost — More than two-dozen buildings across the city are illegally burning dirty heating oil, nearly two months after a deadline to switch to cleaner fuels, authorities said.

Currently, 26 buildings are polluting the air by burning No. 6 oil, a sludge-like substance captured from the bottom of oil barrels, which can lead to health issues such as heart attacks and worsen conditions such as asthma.

All the buildings are using expired permits and had years of notice to make the switch to cleaner heating oils, according to the Department of Environmental Protection.

The number of buildings burning No. 6 oil has dropped significantly since December, when nearly 700 had expired permits.

Source: MAP: Dozens of NYC Buildings Illegally Burning Dirty Oil Past Deadline – Upper West Side – DNAinfo.com New York