Dear Editor,

The COVID-19 pandemic is the first one caused by a human-infecting coronavirus, SARS-CoV-2, which begins its transmission through the epithelial cells of the respiratory tract (^{Hu et al., 2021}). Since the first case reports in Wuhan, China, in January 2020, the World Health Organization (WHO) confirmed person-to-person transmission and declared a pandemic in March (^{WHO, 2020a}). Since the beginning of the pandemic, the WHO stated that the main transmission mechanisms are respiratory droplets produced by coughing or sneezing, as well as fomites, when there is contact with contaminated surfaces (^{WHO, 2020b}). Thus, the recommendations to reduce the transmission of the virus were focused on the detection and isolation of symptomatic people, hand washing, and social or physical distancing. Potential infection from contaminated surfaces motivated the population and health authorities to promote and apply exaggerated cleaning and disinfection measures (Ives, 2021).

In July 2020, a group of 239 scientists pointed out that, within the first six months of the pandemic, there was already evidence of the crucial role of a third transmission mechanism of the virus: aerosols. They called on the medical community, as well as national and international agencies and authorities, including the WHO, to take preventive measures to reduce airborne transmission (^{Morawska etal., 2020}). Despite the evidence provided by the scientific community, the ^{WHO (2021)} recognized the transmission by aerosols only until April 2021. There is still a lack of clarity in the communications of the WHO and health authorities around the world about the importance of the measures that can be more effective in reducing the risk of aerosol transmission, particularly in the evaluation and improvement of ventilation and indoor air filtration.

Aerosols and droplets have different characteristics. Droplets are larger than 100 pm and, due to their size, they rapidly fall to the floor with an average distance of 2 m in seconds or a few minutes after they are exhaled (when speaking, screaming, singing, sneezing, or coughing). On the other hand, aerosols are less than 100 pm in size and are also emitted when breathing or speaking normally. Unlike droplets, aerosols stay floating in the air, as does cigarette smoke, can linger for hours, and accumulate in indoor spaces with poor ventilation (^{van Doremalen et al., 2020}). Understanding these characteristics and accepting that this is the predominant route of SARS-CoV-2 transmission is essential to controlling the pandemic.

The timid and late recognition of aerosol transmission by the WHO was also reflected in the late recognition by governments that aerosols are the main transmission route. Different academic organizations and non-governmental organizations have called upon governments to acknowledge this and act accordingly. Academic groups from Australia (^{Health Care Workers Australia, 2021}), England (^{Royal College of Nursing, 2021}), Spain (^{Aireamos, 2021}), Canada, and the United States, among others, have addressed open letters to the governments of their countries, advocating for the recognition of aerosol transmission and the implementation of preventive measures with an emphasis on the use of high-efficiency face masks in more exposed populations, such as healthcare workers, and the evaluation and monitoring of indoor ventilation conditions.

In Colombia, a group of scholars from the Global Environmental and Occupational Health Hub (Nodo SAO Colombia), in an open letter to the Minister of Health and Social Protection, dated February 2021, requested to take this evidence into account and update Resolution 666 of 2020, which established general protocols without mention of ventilation (^{Nodo SAO Colombia, 2021}). Subsequently, the Ministry issued Resolution 223 of 2021, which included ventilation as one of the four general biosafety measures with the greatest evidence for the containment of transmission. The two previous resolutions were repealed by Resolution 777 of 2021, through which the general biosafety protocol was adopted for the development of economic, social, and State activities, where ventilation is mentioned as one of the transmission control strategies. This, however, was approached in a very general way, indicating the importance of natural and cross ventilation, but without specific guidelines regarding the need to characterize indoor spaces, measure their capacity for air exchange or renewal, and monitor ventilation. Although the measurement of carbon dioxide (CO₂) is currently the best indirect measurement of the quality of indoor ventilation that we have (^{Peng and Jimenez, 2021}), it is not mentioned in the Resolution.

In a commentary published in April 2021 in The Lancet journal, a group of leading researchers provided at least ten scientific reasons for which aerosols the predominant transmission mechanism of SARS-CoV-2 (^{Greenhalgh etal., 2021}). Among others, the evidence shows superspreading events in closed spaces in which physical distancing was maintained and there was no physical contact between people or with surfaces. Evidence for the transmission of SARS-CoV-2 by aerosols is already available, and, after 18 months of pandemic, there is no doubt that the virus is airborne (^{Tang et al., 2021}), so why continue to ignore the importance of ventilation and hesitate in the face of the evidence?

Acting consistently with the fact that SARS-CoV-2 is transmitted through the air implies recognizing and acting on the importance of consistent and appropriate use of face masks and guaranteeing good ventilation conditions in indoor spaces, where air is shared by several people. It also implies informing and educating about the importance of adequate ventilation and how we can evaluate and achieve it (^{Allen and Ibrahim, 2021}). Additionally, in the field of public spaces, informational and educational actions need to be reinforced with clear guidelines on how to evaluate the quality of ventilation and guarantee minimum ventilation conditions that include CO₂ monitoring. As long as these recommendations do not become guidelines, it is very difficult for them to be voluntarily adopted by establishments, institutions, and companies, despite the evident cost-benefit of their implementation in the country. Faced with this situation, some European countries such as Belgium and local governments in Spain have begun to include, within their national guidelines, the obligation to monitor CO₂ in public establishments and make it visible to their occupants with indications of maximum permissible limits (The Irish Times, 2021).

In the return to work and school activities, which mainly occur in closed places, where people spend a significant amount of their time, the control of the transmission of SARS-CoV-2 through adequate ventilation takes on special importance (^{di Gilio et al., 2021}).

Until July 16,2021, more than 4,6 million confirmed cases and more than 115 333 deaths have been recorded in Colombia. In March 2021, the so-called third epidemic peak in the country began, which has been the longest and most severe in terms of the number of cases, deaths, collapse of hospital services, and which has occurred in the midst social unrest and the introduction of new SARS-CoV-2 variants. The government is currently focused on the National Vaccination Plan, which it is undoubtedly a necessary strategy that needs to advance much more quickly in order to protect a larger portion of the population. However, the government should not lose sight of the fact that the protection provided by vaccines mainly avoids severe cases and deaths, but, although the evidence indicates that vaccines can reduce transmission, they do not eliminate it.

Given the amount of people that are still susceptible to infection and the introduction of the ever-increasing SARS-CoV-2 variants in the country, the purpose of the strategies should aim to reduce transmission and not only to identify and treat cases. For all known variants, the main transmission mechanism continues to be through the air, and, therefore, prevention measures are similar and should be based on the consistent and appropriate use of face masks, as well as the evaluation, monitoring, and improvement of ventilation in indoor spaces. For this reason, we call upon citizens, unions, and institutions to assume the importance of adequate ventilation. We ask the national government to take action to inform and educate on the importance of ventilation, and to define clear guidelines to evaluate, guarantee, and monitor the adequate ventilation in interior spaces open to the public.