On November 12, 2024, the U.S. Environmental Protection Agency (EPA) proposed to exempt the compound (Z)-1-chloro-2,3,3,3-tetrafluoropropene (HCFO-1224yd(Z), (CAS No. 111512-60-8) from being classified as a volatile organic compound (VOC) because it has minimal effects on tropospheric ozone formation. Public comments are invited through https://www.regulations.gov/ until January 13, 2025.
This decision is supported by studies of the compound's rate constant with hydroxyl radicals (kOH) and its maximum incremental reactivity (MIR), both of which show significantly lower reactivity than ethane, justifying its VOC exemption.
Tropospheric ozone: formation and effects
Tropospheric ozone, also known as ground-level ozone or smog, is formed when volatile organic compounds (VOCs) and nitrogen oxides (NOx) react in sunlight. Given the health risks of ozone, the US EPA and state governments strictly regulate VOC emissions. However, the varying reactivity of VOCs means that emission reductions do not affect local or regional air quality equally.
Strategic Regulatory Framework
EPA policy since the early 1970s has been focused on excluding less reactive organic compounds from the VOC regulatory scope. This allows for targeted control efforts on compounds that significantly affect ozone levels, and encourages industry to adopt safer, less reactive alternatives.
Empowered by Section 302(s) of the Clean Air Act, EPA defines “VOCs” and identifies which substances are regulated. This approach was first outlined in the 1977 “Policy on the Control of Volatile Organic Compounds” and elaborated in the 2005 “Interim Guidance on the Control of Volatile Organic Compounds in Ozone State Implementation Plans.”
The recent proposal to exempt HCFO-1224yd(Z) is intended to reduce environmental impacts and promote the use of safer alternatives. EPA continues to monitor this compound through programs such as the Supplemental Nutrition Assistance Program (SNAP) and the New Chemicals Program to minimize unanticipated environmental risks.
