Air conditioning, commonly abbreviated as “AC,” is a system or appliance that removes heat from an indoor space, typically through a refrigeration cycle, and transfers that heat outdoors. Modern air-conditioning systems can also dehumidify and filter indoor air. When heating, ventilation, and air conditioning are considered together, the overall system is commonly referred to as HVAC (heating, ventilation, and air conditioning).

Air conditioning has become increasingly widespread around the world, particularly in regions with hot or humid climates. In East Asia, it is now common in countries and regions including Japan, South Korea, mainland China, Hong Kong, and Taiwan. China has experienced especially rapid growth in air-conditioning ownership and cooling demand. The International Energy Agency (IEA) estimates that nearly 90% of households in Japan and the United States have an air conditioner, while air-conditioning ownership in China has risen substantially over the past decade.

Residential systems in East Asia commonly include room air conditioners, ductless mini-split systems, and multi-split systems. These systems can provide efficient cooling to individual rooms or several rooms without requiring the large duct networks used by many North American central-air systems. As incomes, access to electricity, and demand for thermal comfort have increased, air conditioning has also expanded in Southeast Asia. Nevertheless, ownership remains considerably lower there than in countries such as Japan and the United States, despite the region’s substantial need for cooling. The IEA projects continued rapid growth in residential air-conditioning use in Southeast Asia.

In the United States, air conditioning is now widespread across most regions rather than being concentrated primarily on the East Coast and in the South. According to the U.S. Energy Information Administration’s 2020 Residential Energy Consumption Survey, 88% of U.S. households used air conditioning. The South had the highest regional usage rate at 93%, followed by the Midwest at 92%, while usage was lower in the West and Northeast. Central air conditioning or central heat pumps were the main cooling systems for about two-thirds of U.S. households. In Florida, 96% of households used air conditioning in 2020, with 90% using a central air-conditioning unit.

Air conditioning is also increasingly common in newer U.S. homes. Among homes built between 2010 and 2020, 93% used air conditioning in 2020, compared with 83% of homes built before 1950.

In Europe, residential air conditioning has historically been less widespread than in the United States or East Asia, although adoption has been increasing as summers become hotter and heat waves become more frequent. The need for cooling is therefore becoming a more important consideration in European housing and energy policy. The IEA reports that cooling demand has been rising in Europe, with cooling-degree days in 2024 approximately 25% higher than in 2020.

The growth of air conditioning is closely connected with climate change. Rising temperatures and more frequent extreme-heat events increase the need for indoor cooling, while air-conditioning equipment itself consumes electricity and can contribute to greenhouse-gas emissions depending on how that electricity is generated. Refrigerants can also contribute to climate change if they escape into the atmosphere. At the same time, air conditioning can provide important protection during heat waves. The World Health Organization recognizes cooling as an important means of reducing exposure to dangerous heat, particularly for people who are more vulnerable to extreme temperatures.

Air conditioning should not, however, be regarded simply as an air-purification system. Many air conditioners contain filters that can remove some airborne particles, but their effectiveness depends on the type and quality of the filter, the design of the system, airflow, and maintenance. Research has found that air filtration can reduce indoor particulate matter, although improvements in asthma symptoms and lung function are less consistent.

The relationship between air conditioning and asthma is also more complicated than previously suggested. Air conditioning itself is not generally considered a direct cause of asthma. However, cold air, high humidity, mold, dust mites, pollen, and other indoor or outdoor pollutants can trigger asthma symptoms in susceptible people. Properly maintained air conditioning can actually help in some circumstances by reducing indoor temperature and humidity. The U.S. Centers for Disease Control and Prevention (CDC) recommends using air conditioning or dehumidification to help keep indoor humidity low and reduce mold growth, which can trigger asthma attacks.

As air conditioning becomes more widespread, the challenge is therefore not simply whether to cool buildings, but how to provide safe, affordable, energy-efficient cooling while maintaining good indoor air quality. Building design, insulation, shading, ventilation, efficient equipment, appropriate filtration, and low-impact cooling technologies can all play a role alongside air conditioning.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026