Thousands of Long Island businesses are not in compliance with a state law requiring carbon monoxide detectors in all commercial buildings even though the deadline was this week.

Town fire marshals and building inspectors have been working with businesses for the past several months to make sure they are aware of the new regulations, which were signed into law by Gov. Andrew M. Cuomo in December 2014. The law took effect in June 2015, but had a one-year transition period that ended on Monday.

Carbon monoxide is an odorless, colorless gas that can cause sudden illness and death. The Centers for Disease Control and Prevention estimates that each year in the United States, more than 400 people die from carbon monoxide poisoning and 20,000 go to the emergency room.

New York’s law has been dubbed Steven Nelson’s Law after a restaurant manager who died from a carbon monoxide leak in Legal Sea Foods in Huntington Station in February 2014. Dozens of people were sickened from the leak. The building had no detectors.

Last week, five people were made ill by a leak in a West Babylon ShopRite. The store is in the process of installing a detection system, a spokeswoman said.

In Huntington, the fire marshal has been receiving about 15 calls a week from building and business owners asking about the new law, said town spokesman A.J. Carter.

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“The law has multiple ways of compliance,” he said. “It’s 14 pages long and there’s no one size fits all answer.” Carter estimated that of the town’s 4,900 commercial properties, “not many are in compliance.”

For smaller businesses, compliance can be met by installing a plug-in detector that costs about $30, said Babylon Town Deputy Commissioner of Public Safety Patrick Farrell. But for larger businesses with hard-wired smoke alarms, a more complex carbon monoxide detection system must be hard-wired as well. Such systems can cost thousands of dollars, he said.

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“For a place like Tanger [Outlets], it’s a big undertaking,” Farrell said. “Everyone at the end of the day is thinking of safety, but there is a cost to this.”

Farrell said of the roughly 5,000 businesses his department has visited this year, about 300 have been given notice that they are not in compliance.

Keith Williams, senior fire marshal for Brookhaven Town, said nonprofits and other organizations with limited funding have balked because of the expense. “We do have situations where there has been some resistance,” he said, adding that the town will give a time extension in those cases.

Some towns, including Islip and Oyster Bay, report that all of their commercial properties are in compliance.

Other municipalities — Nassau County, Hempstead Town and Brookhaven Town — had existing carbon monoxide laws. Brookhaven’s law is more restrictive than the state’s and does not allow plug-in detectors.

Williams said a “large percentage” of Brookhaven’s commercial buildings are in compliance.

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“We do have the occasional ones that are resistant and have to have an invitation to the court system,” he said. “But for the most part, I think it’s gone fairly well.”