Covid-19 redefined air shipping

Over a year after the covid-19 pandemic, we are still discussing the role and importance of aerosol transfer for SARS-CoV-2, which is only briefly mentioned in some guidelines for infection control.

The confusion stems from the traditional terminology introduced during the last century. This has created poorly defined separations between ‘droplet’, ‘in the air’ and ‘droplet nucleus’ transfer, leading to misunderstandings about the physical behavior of these particles.3 If you inhale particles – regardless of size or name – you are inhaled aerosols. Although this can happen at a long distance, it is more likely if you are near someone, as the aerosols between two people at a short distance are more concentrated, rather than being near someone smoking.

People infected with SARS-CoV-2 produce very small respiratory particles loaded with viruses when they exhale. Some of these will be inhaled almost immediately by those who are ‘short-distance’ within a typical conversation (<1 m), while the remainder disperse over longer distances to be inhaled by others further away (>2 m). Traditionalists refer to the larger short-range particles as droplets and the smaller long-range particles as droplet nuclei, but these are all aerosols because they can be inhaled directly from the air.

Why does it matter? For the current purposes of infection control, it is mostly not. Wearing masks, keeping your distance and reducing indoor occupancy impedes normal transmission routes, either by direct contact with surfaces or droplets, or by inhalation of aerosols. An important difference, however, is that more emphasis should be placed on ventilation, because the smallest suspended particles can remain in the air for hours, and this is an important transmission route.

If we assume that someone in an indoor environment can inhale enough virus to cause infection if it is more than 2 m from the original source – even after the original source is gone – then air replacement or air purification mechanisms become much more important.67 It means to open windows or to install or upgrade heating, ventilation and air-conditioning systems, as set out in a recent WHO document.8 People are much more likely to become infected in a room with windows that can not be opened or which has no ventilation system.

A second important implication of distribution in the air is that the quality of the mask is important for effective protection against inhaled aerosols. Masks usually impede large drops of landing on covered parts of the face, and most are at least partially effective against inhalation of aerosols. However, a high filtration efficiency and a good fit are needed to improve the protection against aerosols, because small particles in the air can find any gaps between mask and face.

If the virus is transmitted only by larger particles (droplets) that fall to the ground within a meter or so after exhalation, it is less of a concern. As health workers wearing surgical masks became infected without being involved in aerosol generation procedures. 111213 As the distribution of SARS-CoV-2 in the air is fully recognized, our understanding of aerosol generating activities will be more necessary. Aerosol scientists have shown that even talking and breathing are aerosol-generating procedures

It is now clear that SARS-CoV-2 is usually transmitted between people at short distances by inhalation. This does not mean that transmission by contact with surfaces or that the long-distance route does not take place, but these transmission routes are less important during short daily interactions over the usual conversation distance of 1 m. In close situations, people are more likely to be exposed to the virus by inhaling it than to let it fly through the air in large drops to land on their eyes, nostrils or lips.17 The transmission of SARS-CoV 2 after contact with surfaces is now considered relatively minimal.181920

Improving indoor air quality through better ventilation will bring other benefits, including reduced sick leave for other respiratory viruses and even environmental complaints such as allergies and morbidity syndrome.2122 Less absenteeism – with its detrimental effect on productivity – can save businesses significant costs 23 which is the cost of upgrading of their ventilation systems would compensate. Newer systems, including air cleaning and filtration technologies, are becoming more efficient

Covid-19 could very well become seasonal, and we will have to live with it as with the flu.25 Governments and health leaders must therefore pay attention to science and focus their efforts on airborne transmission. Safer indoor environments are required, not only to protect people who have not been vaccinated and those for whom vaccines fail, but also to deter vaccine-resistant variants or new threats in the air that may occur at any time. Improving indoor ventilation and air quality, especially in healthcare, work and educational environments, will help everyone stay safe, now and in the future.