Several studies reported an overall decrease in the number of Pediatric Emergency Department (PED) admissions in 2020 during lockdown periods related to social measures adopted to limit spreading of SARS-CoV-2 pandemic, including a marked reduction of asthma referrals^1,2. This led to a growing interest in determining to what extent these life-style changes might have influenced asthma triggers^3–5.

Asthma is a multi-factorial airways disease: in predisposed people, exposure to respiratory infections, air pollution and allergens may trigger asthma exacerbations^6,7. Infections are the main triggers of acute bronchospasm in children of any age, especially in the preschoolers^8,9. Sensitization to environmental allergens, such as pollen, is reported as a further important risk factor for the development of asthma exacerbations¹⁰.

Regarding the role of air pollution, most studies reported how particulate matter smaller than 2.5 microns and 10 microns (PM_2.5, PM₁₀) and the co-emitted gaseous pollutants (NO, NO₂, O₃) can induce airway inflammation, hyper-responsiveness and oxidative injury to the airways, which can lead to asthma¹¹. However, even if the international air quality standards for atmospheric particulate matter (PM) are based on total mass concentrations, the World Health Organization (WHO) acknowledges that not every PM chemical component is equally important in causing airways disease¹². Research on this issue is mostly focused on traffic-related air pollution (TRAP) and has provided evidence of the links between adverse health effects and PM chemical composition^11,13–15. Urban anthropogenic PM presents a three-fold higher oxidative potential per unit of PM mass concentration than rural PM¹⁶.

In this observational retrospective cross-sectional study, we aimed to identify how asthma-related referrals in children were affected by the restrictive measures during the entire 2020, in relation to the main features of potentially associated environmental triggers, i.e. air pollution and pollen. The selected urban area of Bologna, in Northern Italy, represents an important population basin of about 400000 inhabitants and can be considered an ideal setting for this study as it is one of the areas more dramatically hit by the pandemic, and also well-known for a high prevalence of respiratory disease and for being one of the main air quality European hotspot, characterized by PM levels well above the limit set by the European Air Quality Directive and by the WHO.