Coronavirus disease 2019 (COVID-19), caused by the novel betacoronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has so far led to over 171.2 million infections and over 3.56 million deaths.
Attempting to understand why the SARS-CoV-2 has spread across the globe, persisted for long periods in the environment, and has high contagiousness, researchers have demonstrated an association between the chronic exposure to some air pollutants and the transmission and severity of the SARS-CoV-2 infection.
Atmospheric pollutants produce oxidative stress, inflammation, immuno-unbalance, and systemic coagulation, rendering the human body prone to infections by various pathogens. On the other hand, it is known that spermatozoa (male gametes/sperm cells) are extremely responsive to pro-oxidative effects produced by environmental pollutants. Thus, these may serve as powerful alerts that signal the extent of environmental pressure in a specific area is doing damage to humans.
In a recent review in Environmental Science and Pollution Research, Italian researchers focused on “the evidence that correlates the degree and type of pollution with the increased susceptibility of many countries to this pandemic and proposes human semen as an early marker of the environmental health and the general health of individuals.” Their observations are based on a robust body of experimental evidence.
Air pollution and COVID-19
In regions where increased air particulate matter (PM) is reported, a high incidence of COVID-19 cases is observed. Many studies report a significant association between elevated levels of PM2.5, carbon monoxide (CO), nitrogen dioxide (NO2), and COVID-19 diffusion and mortality.
The researchers write:
For example, a study performed in Italy indicated that more than 75% of infected individuals and about 81% of deaths in the first wave of the COVID-19 pandemic in Italy happened in industrialized regions with high levels of air pollution.”
Interestingly, the researchers specified the various environmentally-related factors that linked to the number of people infected with SARS-CoV-2 during the first wave of the pandemic: 1) winter time and increased related air pollution, 2) PM10, in areas increasing the set limits, 3) ozone, in cities that crossed the limit for 100-plus days per year, 4) low average wind speeds, and 5) lower average temperatures. They reported that there were more than three times as many infected cases when compared with the relative number in cities with low levels of air pollution.
In the review, the researchers retrace the path of the SARS-CoV-2 transmission from China (where it was first identified in December 2019) to South Korea, Iran, Italy and the rest of the world, along with the climatic and weather conditions which were propitious for the spread of the virus.
Pollution and susceptibility to viral insults
According to the World Health Organization (WHO) reports, approximately a quarter of diseases, including cardiovascular and chronic degenerative disorders, early deaths, and reproductive impairments, are caused by exposure to environmental pollutants over time, together with lifestyle. Environmental pollution enhances the susceptibility to non-communicable disease (NCD).
The consequence is a decline in the human immune defenses, also attributed to transgenerational effects, which reduce the ability to fight viral pathogens.
A recent study in mice shows chronic exposure to PM2.5 resulted in increased expression of the angiotensin-converting enzyme 2 (ACE2), which is the human host cell receptor that facilitates SARS-CoV-2’s entry.
Pre-existing environmental factors alter mechanisms in the immune responses that may hinder the ability to fight pathogens effectively.
The team writes:
In addition to this, it should be added that air pollutants represent a potential co-factor of major damage because they are capable of inducing oxidative stress, inflammatory processes, immune imbalance, and coagulation at the systemic level.”
Interestingly, a positive correlation between NO2 and COVID-19 is observed in countries such as Europe: France, Italy, Spain, and Germany. NO2 enhances ACE2 expression, and SARS-CoV-2 interacts through ACE2 with the renin-angiotensin-aldosterone system (RAAS), which controls blood pressure and fluid and electrolyte balance, and also systemic vascular resistance.
Sperm decline in polluted areas
Sperm decline is observed in industrial development with high levels of air pollution. A wide range of environmental stressors (that result in oxidative stress) alters the seminal parameters such as sperm count, motility, morphology, and especially the integrity of sperm DNA.
Based on previous studies, the researchers confirmed that “The negative trend in sperm quality in conjunction with annual high average levels of PM10, PM2.5, and NO2, could suggest that sperm decline may be the first clinical sign of environmental pressure and that semen quality could be a potential indicator of susceptibility to insults in polluted areas including viral infections as described in the following section.”
Human semen as environmental and health marker
Our studies on individuals living in polluted areas demonstrated that also human semen could be considered an ideal early sentinel with a double function: environmental and human health,” said the researchers in the review.
Human semen is a “sentinel biomarker” of subclinical biological effect suitable for monitoring the impact of adverse environmental exposures, and so could also support the comprehension of the dynamics involved in the facilitation of COVID-19 severity in polluted areas, suggested the researchers.
Heavy metals adversely impact sperm cells. A recent study shows an association between heavy metal exposure and the severity of viral diseases, including influenza and respiratory syncytial virus. Thus, early signs of damage to organ-sentinel systems (such as the male reproductive system/human sperm quality) may hint at how relevant environmental pressures are.
In conclusion, the researchers suggest, through the use of human semen, as an early sentinel of environmental and general human health status, the population health status in a given environmental context can be known. This can help predict both a population’s susceptibility to a virus’s impact and the medium and long-term negative effects it may cause on human health.
The researchers conclude:
We also believe that semen quality used as an early environmental and health marker could help policymakers to promptly intervene in areas with significant environmental criticality to abate air, water, and soil pollution with an integrated One Health approach, where information sharing among diverse and key professionals (clinicians, biologists, chemists, virologists, veterinarians, economists, epidemiologists) could be successful in establishing a systemic approach that could be efficient and beneficial on a global scale.”