The American built environment's reliance on heating and cooling systems has undergone substantial evolution since the early 20th century. Initially, natural refrigerants such as ammonia, carbon dioxide, and hydrocarbons were widely used in cooling systems. These natural refrigerants were effective in terms of refrigeration and air conditioning but posed significant safety risks, including toxicity and flammability.

The advent of synthetic refrigerants in the 1920s and 1930s marked a revolutionary shift in the industry. Chlorofluorocarbons (CFCs), marketed under names like Freon, were developed by Thomas Midgley Jr. and his team at General Motors. CFCs were celebrated for being non-toxic, non-flammable, and highly efficient, leading to their rapid adoption in residential and commercial air conditioning, as well as in refrigeration systems. This transition significantly contributed to the proliferation of HVAC systems in homes, offices, and public buildings across the United States, profoundly impacting the built environment by enhancing indoor comfort and air quality.

However, by the 1970s, the environmental drawbacks of CFCs became evident. Researchers discovered that CFCs were major contributors to ozone layer depletion. The ozone layer protects life on Earth from harmful ultraviolet (UV) radiation. In response to this discovery, the international community, under the guidance of the United Nations, formulated the Montreal Protocol in 1987. This landmark agreement aimed to phase out the production and consumption of ozone-depleting substances (ODS), including CFCs. The success of the Montreal Protocol led to the development and adoption of hydrochlorofluorocarbons (HCFCs) as interim replacements for CFCs. While HCFCs were less harmful to the ozone layer, they were still problematic due to their global warming potential (GWP).

In 2016, the Kigali Amendment to the Montreal Protocol was adopted, targeting the reduction of hydrofluorocarbons (HFCs), the successors to HCFCs. Although HFCs do not deplete the ozone layer, they possess high GWPs, contributing significantly to global warming. The amendment aimed to reduce HFC production and consumption by more than 80% over the next 30 years, potentially preventing up to 0.4°C of global temperature rise by the end of the century.

As a result of these international agreements and growing environmental awareness, the HVAC industry has been transitioning back to natural refrigerants and developing new synthetic alternatives with lower GWPs. This shift is driven by advancements in HVAC technology and a renewed focus on energy efficiency and sustainability. Modern natural refrigerants, such as carbon dioxide (CO2), ammonia (NH3), and hydrocarbons (like propane and isobutane), are being engineered to mitigate safety risks while offering low environmental impact.

Moreover, the American Innovation and Manufacturing (AIM) Act of 2020 further bolstered efforts to manage refrigerants in the U.S. This legislation provides a framework for phasing down the production and consumption of HFCs, promoting the use of climate-friendly refrigerants, and supporting recovery, recycling, and reclamation programs.

Today, the focus on reducing refrigerant emissions is integral to broader climate change mitigation efforts. Transitioning to low-GWP refrigerants and enhancing HVAC system efficiency are critical in reducing the carbon footprint of the built environment. These efforts contribute to global initiatives aimed at curbing global warming and protecting the planet for future generations.

For more detailed insights, you can explore resources like the Environmental Protection Agency's historical overview of refrigerants and the American Society of Heating, Refrigerating and Air-Conditioning Engineers' (ASHRAE) publications on HVAC advancements.