Abstract
Outdoors, the risks of transmission of COVID-19 and many other respiratory infections are low. Several environmental factors are known to reduce the viability of viruses and other infectious pathogens in the air. They include variations in temperature, relative humidity, solar ultraviolet radiation, and dilution effects. But one agent that reduces the viability of both viruses and bacteria outdoors, the germicidal open-air factor (OAF), has not been properly recognized for decades. This is despite robust evidence that the OAF can influence both the survival of airborne pathogens and the course of infections. The germicidal effects of outdoor air were widely exploited during the late 19th and early 20th centuries. Firstly, in the treatment of tuberculosis patients who underwent 'open-air therapy' in sanatoria; and secondly by military surgeons during the First World War. They used the same open-air regimen in specially designed hospital wards to disinfect and heal severe wounds among injured soldiers. It was also used on influenza patients during the 1918-19 pandemic. Later, in the 1950s, open-air disinfection and treatment of burns were proposed in the event of nuclear warfare. During the 1960s, the OAF briefly returned to prominence when biodefence scientists conducted experiments proving that open air has a potent germicidal effect. When this work ended in the 1970s, interest in the OAF again fell away, and it remains largely ignored. The COVID-19 pandemic has revived interest in understanding the transmission dynamics and survival of viruses in the air. The pandemic has also stimulated research in the science and practice of improved ventilation to control respiratory infections. Such work is incomplete without an appreciation of the inactivation of viruses and other pathogens by the OAF, but this needs further investigation as a matter of urgency. Research to better understand the conditions under which the OAF can be preserved indoors is urgently needed. We need to review building design with better regard to infection control and patient recovery. But we need to act without delay, as there is already sufficient evidence to show that public health generally would improve if more emphasis was placed on increased exposure to outdoor air.