Is there a Plausible Role for Particulate Matter in the spreading of COVID-19 in Northern Italy?
Air pollution in the Po Valley, especially particulate matter (PM) has received increasing attention in the past years, since exposure to PM2.5 and PM10 is notoriously associated with a number of adverse health effects, ranging from respiratory and cardiovascular to neurological and metabolic diseases or even premature death.  At the same time, a number of studies have shown that airborne transmission route could spread viruses even further the close contact with infected people. [2-15] An epidemic model based only on respiratory droplets and close contact could not fully explain the regional differences in the spreading of the recent severe acute respiratory syndrome COVID-19 in Italy, which was fast and dramatic only in Lombardy and the Po Valley.
The higher mortality rates associated with COVID19 observed in these areas, already characterized by atmospheric stability and high humidity, were proposed on March 16th 2020 in the Italian Society of Environmental Medicine (SIMA) Position Paper to be associated with the peaks of fine particulate matter concentrations, frequently exceeding the legal limit of 50 µg/m3.  A significant correlation was found between the geographical distribution of the daily PM10 exceedances in 110 Italian Provinces and the spreading of the COVID-19 infection during the time-lapse of the study, with the number of PM10 exceedances being much more frequent in Lombardy and in cities located in the Po valley than those registered in Rome and Southern Italy, where the diffusion and lethality of the virus was significantly lower if compared with that observed in Northern regions.  Research carried out by the Harvard School of Public Health seems to confirm an association between increases in particulate matter concentration and mortality rates due to COVID-19. 
These first observations suggest that particulate matter could be regarded as an indicator of the severity of COVID-19 infection in terms of diffusion and health outcomes. Further experimental studies could assess the possibility that particulate matter may act as a “carrier” for the viral droplet nuclei, impressing a boost effect for the spreading of the viral infection, as it has been shown for other viruses.
Indeed, Paules et al. (2020) highlighted that airborne transmission of SARS-CoV can occur , besides close distance contacts. For different pathogens, it has also been reported how they can reach long distances thanks to airborne transport [5-7]. Reche et al. (2018) described the aerosolization of soil-dust and organic aggregates in sea spray that facilitates the long-range transport of bacteria - and likely of viruses - free in the atmosphere. In particular, virus deposition rates were positively correlated with organic aerosol <0.7 µm, implying that viruses could have longer permanence times in the atmosphere and, consequently, will be dispersed further .
Moreover, Qin et al. (2020) analyzed the microbiome of the airborne particulate matter (PM2.5 and PM10) in Beijing over a period of 6 months in 2012 and 2013, putting in evidence a variability of the composition that depended on the months . Temporal distribution of the relative abundance of the microbiome on the particulate matter (PM) showed the highest presence of viruses in January and February, just in coincidence with highest concentration of particulate matter. Chen. et al (2017) demonstrated the relationship between short-term exposure to PM2.5 concentration and measles incidence in 21 cities in China . Their meta-analyses showed that the nationwide measles incidence was significantly associated with an increase of 10 µg/m3 in PM2.5 levels.
Other recent studies have also reported associations between particulate matter concentrations and infectious diseases (e.g., influenza, hemorrhagic fever with renal syndrome) because inhalation could bring PM deep into the lung and virus attached to particles may invade the lower part of respiratory tract directly, thus enhancing the induction of infections, as demonstrated by Sedlmaier et al. in 2009 . Zhao et al. (2018) showed that the majority of the positive cases of highly pathogenic avian influenza (HPAI) H5N2 in Iowa (USA) in 2015 might have received airborne virus, carried by fine particulate from infected farms both within the same State and from neighboring States . Ma et al. (2017) observed a positive correlation of the measles incidence with PM10 in western China during the period 1986-2005. 
The condensation and stabilization of the bio-aerosol, generating aggregates with atmospheric particles from primary (i.e. dust) and secondary particulate, has been indicated as mechanisms able to transport airborne bacteria and viruses to distant regions, even by the inter-continent-transported dust.  Ferrari et al. (2008) described measles outbreaks occurring in dry seasons and disappearing at the onset of rainy seasons in Niger , but Brown et al. had already found that the most severe measles epidemic in the United States occurred in Kansas in 1935 during the Dust Bowl period . Coming to recent specific studies, laboratory experiments of Van Doremalen et al. (2020) indicated that airborne and fomite transmission of SARS-Cov-2 is plausible, since the virus can remain viable and infectious in aerosol for hours .
Based on the available literature, there is enough evidence to consider the airborne route, with a possible role of particulate matter, as a possible additional infection “boosting” factor for interpreting the anomalous COVID-19 outbreaks observed in Northern Italy – known to be one of the European areas characterized by the highest PM concentration . Therefore, there is the rationale for carrying out experimental studies specifically aimed at confirming or excluding the presence of the SARS-CoV-2 (and its potential virulence) on particulate matter of Italian cities as well as at European and international level. Urgent actions must be adopted to counteract climate changes and the alteration of ecosystems that might trigger new and unexpected threats to human health such as that of COVID-19, which we are so dramatically experiencing worldwide.
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Competing interests: No competing interests