The COVID-19 disease spread at different rates in the different countries and in different regions of the same country, as happened in Italy. Transmission by contact or at close range due to large respiratory droplets is widely accepted, however, the role of airborne transmission due to small respiratory droplets emitted by infected individuals (also asymptomatic) is controversial. It was suggested that outdoor airborne transmission could play a role in determining the differences observed in the spread rate. Concentrations of virus-laden aerosol are still poorly known and contrasting results are reported, especially for outdoor environments. Here we investigated outdoor concentrations and size distributions of virus-laden aerosol simultaneously collected during the pandemic, in May 2020, in northern (Veneto) and southern (Apulia) regions of Italy. The two regions exhibited significantly different prevalence of COVID-19. Genetic material of SARS-CoV-2 (RNA) was determined, using both real time RT-PCR and ddPCR, in air samples collected using PM₁₀ samplers and cascade impactors able to separate 12 size ranges from nanoparticles (diameter D < 0.056 µm) up to coarse particles (D > 18 µm). Air samples tested negative for the presence of SARS-CoV-2 at both sites, viral particles concentrations were <0.8 copies m⁻³ in PM₁₀ and <0.4 copies m⁻³ in each size range investigated. Outdoor air in residential and urban areas was generally not infectious and safe for the public in both northern and southern Italy, with the possible exclusion of very crowded sites. Therefore, it is likely that outdoor airborne transmission does not explain the difference in the spread of COVID-19 observed in the two Italian regions.

正如意大利所发生的那样，COVID-19 疾病在不同国家和同一国家的不同地区以不同的速度传播。由于大的呼吸道飞沫通过接触或近距离传播已被广泛接受，但是，由于感染者（也无症状）散发的小呼吸道飞沫引起的空气传播的作用是有争议的。有人提出，室外空气传播可以在确定观察到的传播率差异方面发挥作用。载有病毒的气溶胶浓度仍然鲜为人知，并且报告了对比结果，特别是对于户外环境。在这里，我们调查了 2020 年 5 月在意大利北部（威尼托）和南部（普利亚）地区大流行期间同时收集的含病毒气溶胶的室外浓度和大小分布。这两个地区的 COVID-19 流行率存在显着差异。使用实时 RT-PCR 和 ddPCR 在使用 PM 收集的空气样本中确定 SARS-CoV-2 (RNA) 的遗传物质_{10 个}采样器和级联冲击器能够分离 12 种尺寸范围，从纳米颗粒（直径 D < 0.056 µm）到粗颗粒（D > 18 µm）。空气样本在两个地点都检测出 SARS-CoV-2 的存在呈阴性，PM _{10中的病毒颗粒浓度 <0.8 拷贝 m} ^-3 ，在所研究的每个尺寸范围内<0.4 拷贝 m ^-3。对于意大利北部和南部的公众来说，住宅和城市地区的室外空气通常不具有传染性和安全性，可能会排除非常拥挤的场所。因此，室外空气传播很可能无法解释在意大利两个地区观察到的 COVID-19 传播的差异。