Global lockdown, pollution, and respiratory allergic diseases: Are we in or are we out?

To the Editor:

We have read with great interest the work by Navel et al¹ about the impact of the current coronavirus disease 2019 pandemic on the burden of allergic diseases. The authors suggest that, thanks to the global lockdown of the majority of human industrial activities, the substantial reduction in air pollution in the urban areas, a well-known inducer of nonspecific airway inflammation, may have a positive effect on seasonal allergic diseases.¹ Indeed, we are now facing a unique chance to decipher some of the complex interplays between the outdoor allergens, the air quality, and respiratory allergic diseases but, in order to have a full picture, we believe more (f)actors are needed on the stage.²

It is true that, especially in the most industrialized areas, the atmospheric concentration of some greenhouse gases, mostly nitrogen dioxide (NO₂), has substantially decreased when compared with the previous years (Fig 1 ).³ In the same period, we are facing the usual raise in the concentrations of outdoor aeroallergens such as seasonal pollens: clinical and experimental research has shown that such molecules can interact by several mechanisms with particulate matters and other chemicals, so as to ultimately exacerbate allergic symptoms.^{4 , 5} For instance, pollutants can act as adjuvants by binding to allergens and they can increase the IgE production.⁵ However, the expected benefits of improved outdoor air quality on the exacerbation of allergic disorders may also be linked to other phenomena that should be separately analyzed. First, a near-universal face masking policy has been implemented in the vast majority of the countries since the beginning of the pandemic. Although primarily worn to protect themselves and others from severe acute respiratory syndrome coronavirus 2 transmission, masks are also able to reduce the airway exposure to inhaled aeroallergens depending on the presence and on the type of a filter.⁶

Fig 1.

Data on the 2020 concentration of atmospheric NO₂ and the annual trend in the Lombardy region, Italy.³

Second, the urgent need for social distancing has prompted most of the world’s population to be confined at home. We must not forget that indoor air can also be full of several biological aeroallergens (such as those derived from molds or domestic pets) and chemical toxicants (tobacco, cooking smoke, or cleaning products, which, ironically, are being used more often to achieve environmental sanitization).^{2 , 7} When inhaled, such molecules have also been shown to favor oxidative stress of the respiratory mucosa and, thus, to exacerbate inflammatory conditions such as asthma or allergic rhinitis.^{2 , 7} Therefore, the important changes not only in outdoor air quality and pollution but also in our current forcefully indoor lifestyle may yield paradoxical effects on the allergic population. When, in the next years, we will retrospectively analyze the overall effect of this unprecedented situation on the global burden of allergic and respiratory diseases, it is necessary also to include real-world experimental data on the aforementioned factors. We believe that only in this way we can gather robust evidence and finally appreciate the real consequences of the pandemic in this field.