The Environmental Protection Agency is warning homeowners about an illegally marketed refrigerant for central air conditioners that poses a safety hazard to them and to the technicians that service their systems. Called R-22a or just 22a, the propane-based refrigerant can catch fire or explode. The EPA prohibits its use, which has resulted in enforcement actions—arrests and fines—against those who market and sell it.

While you may not know what refrigerant your HVAC contractor uses to recharge your system, it’s a good idea to ask the next time you get an annual tune-up. One of the most common refrigerants, HCFC22 or just R22, is being phased out because it depletes ozone from the atmosphere. The Environmental Protection Agency has a list of more environmentally friendly alternatives on its website but R-22a isn’t one of them.

“Using an unapproved, flammable refrigerant in a system that wasn’t designed to address flammability can lead to serious consequences, including explosion or injury in the worst cases,” said Janet McCabe, acting assistant administrator for EPA’s Office of Air and Radiation. “As the summer cooling season gets started, we want to make sure consumers and equipment owners know what is going into their system is safe.”

In the past year, the EPA has taken enforcement actions against at least three offenders including assessing civil penalties against two companies for $100,000 and $300,000 and working with the FBI to arrest a Louisiana man for selling a product called “Super-Freeze 22a” to unwitting technicians and homeowners. In one instance, according to the EPA, “the product caught fire, burned, and injured a technician.”

Owners of central air conditioning systems should have them serviced by a pro at least once a year. To find one in your area, check the website of the ACCA, a trade organization. Ask the technician to change all filters, clean and flush the coils, drain the pan and drainage system, and vacuum the blower compartments. The contractor should also check that the system is properly charged with refrigerant, that there are no leaks, and that all mechanical components are working properly.

If they're not and you're in the market for a new central air system, check our central air reliability Ratings.