Assessment of the potential role of PM2.5/PM10 particles in intensifying the pandemic spread of SARS-CoV-2/COVID-19 in Northern Italy
Authors: Girolamo Paolo Di 
Published: September 2020

Abstract
The Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2), which exploded in Wuhan (Hebei Region, China) in late 2019, has recently spread around the World, causing pandemic effects on humans. Italy, and especially its Northern regions around the Po Valley, has been facing severe effects in terms of infected individuals and casualties (more than 31.000 deaths and 255.000 infected people by mid-May 2020). While the spread and effective impact of the virus is primarily related to the life styles and social habits of the different human communities, environmental and meteorological factors also play a role. Among these, pollution from PM2.5/PM10 particles, which may directly impact on the human respiratory system or act as virus carrier, thus behaving as potential amplifying factors in the pandemic spread of SARS-CoV-2. Enhanced levels of PM2.5/PM10 particles in Northern Italy were observed over the two month period preceding the virus pandemic spread. Threshold levels for PM10 (<50 µg/m³) were exceeded on 20-35 days over the period January-February 2020 in many areas in the Po Valley, where major effects in terms of infections and casualties occurred, with levels in excess of 80 µg/m³ occasionally observed in the 1-3 weeks preceding the contagious activation around February 25th. Threshold values for PM2.5 indicted in WHO air quality guidelines (<25 µg/m³) were exceeded on more than 40 days over the period January-February 2020 in large portions of the Po Valley, with levels up to 70 µg/m³ observed in the weeks preceding the contagious activation. The evolution of particle matter concentration levels throughout the month of February 2020 was carefully monitored and results are reported in the paper.In this paper PM10 particle measurements are compared with epidemiologic parameters data. Specifically, a statistical analysis is carried out to correlate the infection rate, or incidence of the pathology, the mortality rate and the case fatality rate with PM concentration levels. The study considers epidemiologic data for all 110 Italian Provinces, as reported by the Italian Statistics Institute (ISTAT, 2020), over the period 20 February-31 March 2020. Corresponding PM10 concentration levels were collected from the network of air quality monitoring stations run by different Regional and Provincial Environment Agencies, covering the period 15-26 February 2020. The case fatality rate is found to be highly correlated to the average PM10 concentration, with a correlation coefficient of 0.89 and a slope of the regression line of (6.7±0.3)×10-3 m³/µg, which implies a doubling (from 3 to 6 %) of the mortality rate of infected patients for an average PM10 concentration increase from 22 to 27 μg/m³. Infection and mortality rates are also found to be correlated with PM10 concentration levels, with correlation coefficients being 0.82 and 0.80, respectively, and the slopes of the regression lines indicating a doubling (from 1 to 2 ‰) of the infection rate and a tripling (from 0.1 to 0.3 ‰) of the mortality rate for an average PM10 concentration increase from 25 to 29 μg/m³. Epidemiologic parameters data were also compared with population density data, but no clear evidence of a mutual correlation between these quantities was found. Considerations on the exhaled particles' sizes and concentrations, their residence times, transported viral dose and minimum infective dose, in combination with PM2.5/PM10 pollution measurements and an analytical microphysical model, allowed assessing the potential role of airborne transmission through virus-transmitting PM particles, in addition to droplet transmission, in conveying SARS-CoV-2 in the human respiratory system.