A Covanta Holding Corp. subsidiary is partnering with a state agency and another firm to provide a recycling option for mercury-containing thermostats in Oklahoma.

Citizens and contractors can deliver intact old thermostats to any Locke Supply location for recycling free of charge. Locke Supply, a heating and cooling distributor, is working with Covanta Tulsa Renewable Energy LLC and the Oklahoma Department of Environmental Quality (ODEQ) on the project, according to a news release.

The used devices are collected at Locke and then sent to Thermostat Recycling Corp., an industry-funded nonprofit organization based in Alexandria, Va. Participants also can contact their local heating, ventilation and air conditioning (HVAC) contractor to properly recycle the thermostats.

Mercury thermostats have been one of target items for producer responsibility laws and programs. It has been a focal point of the Boston-based Product Stewardship Institute.

In late 2013 New York adopted a producer responsibility law for the thermostats. In May of that year California approved mercury thermostat regulations that set high targets for recycling and collecting used thermostats. Ten other states have thermostat recycling laws.

The average wall thermostat contains four grams of mercury, or the equivalent of 800 compact fluorescent lights. Mercury is a heavy metal that can be a threat to human health and the environment.

A report on thermostat recycling in April 2013, “Turning up the Heat II,” reported that Thermostat Recycling from 2002 to 2011 captured only 5.8 percent to 8 percent of the mercury thermostats at the end of their use. The low recycling rate resulted in more than 50 tons of mercury waste being disposed into the environment.

The report characterized the laws in Maine and Vermont as the most effective.

The U.S. Environmental Protection Agency (EPA) has estimated that 2 million to 3 million thermostats reach the end of their use each year in the United States, amounting to 7 to 10 tons of mercury annually.