In response to global warming, human beings have been forced to adapt in response to the long-term warming trend. Long-term warming is concomitant with severe weather events such as heatwaves, cold spells, extreme rainfalls, or storms. One adaptation to extreme heat events is the installation of air conditioning (along with heating and ventilation) in homes and public buildings across the world.

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What is known about the health effects of air conditioning?

Current knowledge of air conditioning-related effects on health is based on studies conducted on office workers or an exercise-related heat illness. Moreover, artificial experimental designs on climatic influences on physiology have also been conducted.

Despite this, studies on the effects of air conditioning on health are scarce and there is an absence of knowledge on how the mechanisms affect patient health.

To investigate the available literature, a paper published in BMC public health analyzed the available evidence published in Medline, Embase, and the Cochrane Library. This review focused specifically on patients in hospitals and results were charted according to the intervention, population, control, and outcome.

This literature also investigated any studies that examined the effect of heating and ventilation alongside air conditioning. Collectively, the publication refers to these modes of air and temperature adaptation as Heating, Ventilation, Air Conditioning (HVAC).

Overall, HVAC was found to improve recovery, notably by improving vital signs, reducing incidents of cardiac stress, accelerating recuperation, and aiding greater physical activity. Moreover, reduced mortality for heat illness patients resulted from the use of HVAC, and resulted in shorter hospital stays for patients with respiratory disease.

Therefore, in the inpatient context, installing HVAC has the potential to improve patient outcomes and increase the efficiency of hospital treatment. This is particularly notable in mitigating the effects of climate change on health and healthcare systems.

Air conditioning in the context of epidemics and pandemics

Since the COVID-19 pandemic, the mode by which the SARS-CoV-2 virus spreads has been studied extensively. It is known that the virus is transmitted primarily through close contact transmission specifically when an infected person coughs, talks, sneezes, or otherwise uses their voice at short distances <1–2 m. The airborne transmission of droplet nuclei (aerosolized particles of <5 μm in diameter) is still controversial and some people have been categorized as super emitters, capable of releasing viral particles during normal breathing and speech, coughing, and talking.

Moreover, the airborne transmission of the virus can occur during medical procedures that produced aerosols. As such, preventative measures have been installed. The transmission capacity of aerosol-generating procedures is also dependant on particulates as they can release toxic molecules and microorganisms which include viruses. As such, ventilation systems including heating and air conditioning which involve the recirculation of exhausted air have been subject to review.

Alongside respiratory viruses (influenza and coronaviruses), other microorganisms such as vegetative bacteria (staphylococci and legionellae), fungi, (Aspergillus and Penicillium), mycobacteria (tuberculous and nontuberculous), enteric viruses (noro- and rotaviruses), and bacterial spore formers (Clostridium difficile and Bacillus anthracis), can also be spread by air. The virality of such pathogens is dependent on the air temperature, relative humidity, and turbulence, among other factors.

As such, air conditioning can spread these pathogenic bacteria, especially when air conditioners do not have an efficient filtration system. Moreover, the risk of air conditioning-caused infections increases when the size of room ventilated increases (making it high in public places like department stores, airplanes, ships, and hospitals).

This is exacerbated as air conditioners do not cool hot air coming from the outside, but rather recirculate indoor air to keep this cool. This results in significantly reduced air exchange while increasing the concentration of pollutants and infectious agents that can exacerbate their pathogenic activity owing to the favorable effects of artificially cold and dry air.

Air  conditioning impact on respiratory health

Air conditioning can result in heightened clinical discomfort in those who last suffer from respiratory illnesses. In cases of chronic respiratory illness, there is a risk of worsening symptoms. Moreover, respiratory infections may be caused by cold air because of increased bronchial inflammation which works alongside other trigger factors such as infection, pollutant inhalation, cigarette smoke, and irritants present in the air.

In healthy individuals, exposure to air conditioners with very cold air can cause alterations in the respiratory airways that, in the context of pre-existing conditions such as asthma, can increase the risk of developing a respiratory illness.

In conditions where the temperature of the air drops quickly, even for small changes between but more markedly for changes more than 5 °C, there is a possible negative consequence exerted on the respiratory system. This effect is most pronounced in those with obstructive respiratory illnesses such as asthma and COPD. Cold air is also known to trigger bronchoconstriction in asthmatics

The effect of air conditioners on cognitive performance

Research from the Harvard Chan school of public health demonstrated that increased indoor temperatures during a heatwave produced detrimental cognitive effects in a group of young healthy individuals without air conditioning. The study followed 44 students from a university in Massachusetts living in air-conditioned and non-air-conditioned buildings before, during, and after a heatwave.

The participants were subject to two cognition tests and the effect of the heatwave on the cognitive function was evaluated. The group found that cognitive function deficits resulted from indoor thermal conditions caused by heatwaves in non-AC buildings. The researchers suggested that this decline in cognitive function could be attributed to increased thermal load and exacerbated effects of other environmental and behavioral factors.

This corroborates existing experimental, epidemiological, and econometric studies that have shown that heat exposure can negatively affect productivity, learning ability, mortality, and mobility in humans.

Other positive impacts of air conditioners on health

Air conditioning critically maintains body temperature. This is critical in humid and hot areas of the world where heatstroke is a sizeable problem during peak summer. Although humans can acclimatize to hot environments, this typically occurs over a long period (7 to 14 days with at least two hours of daily heat exposure) and it is possible for people to lose heat acclimatization when spending the majority of their time in air-conditioned environments. They subsequently find it more difficult to reacclimatise in the future.

Air conditioners are also useful in aiding sleep through the triggering of homeostatic mechanisms to cool the body. Air conditioners can also decrease the incidence of allergic reactions as, if the air conditioners are confined to small spaces in which air is exchanged with the outside, any allergens can be cleared.

References:

• Lenzer B, Rupprecht M, Hoffmann C, et al. (2020) Health effects of heating, ventilation and air conditioning on hospital patients: a scoping review. BMC Public Health. doi: 10.1186/s12889-020-09358-1.
• Chirico F, Sacco A, Bragazzi NL, et al. (2020) Can Air-Conditioning Systems Contribute to the Spread of SARS/MERS/COVID-19 Infection? Insights from a Rapid Review of the Literature. Int J Environ Res Public Health. doi:10.3390/ijerph17176052.
• Cedeño Laurent JG, Williams A, et al. (2018) Reduced cognitive function during a heat wave among residents of non-air-conditioned buildings: An observational study of young adults in the summer of 2016. PLoS Med. doi: 10.1371/journal.pmed.1002605.
• D’Amato M, Molino A, Calabrese G, et al. (2018) The impact of cold on the respiratory tract and its consequences to respiratory health. Clin Transl Allergy. doi: 10.1186/s13601-018-0208-9.

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