• Study in mice indicates pollution-borne pathogens can reach deep into the lungs and attack more distant organs through the bloodstream
• Researchers said results give ‘further urgency’ to air quality management and pollution reduction policies

Air pollution can “act as a Trojan horse” and worsen the severity of viral infections by driving viruses deep into the lungs and other organs, according to a study led by researchers from the Chinese Academy of Sciences (CAS).

“Our results … provide further urgency for prioritising air quality management and air pollution reduction policies,” the researchers said, in a paper published in June by peer-reviewed journal Science Advances.

Co-author Fang Min, a researcher with the academy’s Institute of Microbiology, said that in addition to the harmful effects of air pollution particles on human health, they could also be loading up with pathogenic microorganisms that cause infections.

The researchers observed that particle-borne viruses could travel deep into the respiratory system of mice and reach as far as more distant organs, such as the liver, spleen and kidneys.

Fang said that, while the team observed the phenomenon in animal experiments, it may also apply to humans because “the pathways by which animals are infected with pathogenic microorganisms are similar to those of humans”.

“On smog days, we can easily catch a cold or have severe symptoms … because the viruses are absorbed into the air pollution particles, which facilitates the spread of the virus,” she said.

While scientists have studied the impact and role of air pollution particles in the spread and infection of viruses, how the particles interact with the pathogens and the infection process remains largely unknown.

Fang and her colleagues investigated four common air pollution particles – dust, biochar (wood burnt without oxygen), black carbon, and the fine particulates of less than 2.5 microns known as PM2.5 – and their effects on the transmission and infectiousness of H1N1 viruses.

The team combined the viruses with each of the pollutants in vitro and found “robust and sustained” interactions. Virus loading was highest in the PM2.5 sample, followed by biochar, the study said.

The researchers said their observations showed the particles “acted as a Trojan horse for viral entry into host cells at the early stages of infection”.

The team also exposed mice to environments that included a mixture of each pollutant and the H1N1 viruses, as well as a control group. The findings indicate that different types of air pollution have different viral biodistribution patterns.

The study found 82 per cent of viruses carried by PM2.5 particles ended up in the lungs of the mice, compared with 76 per cent for biochar.

The researchers also found that dust – while less effective at absorbing the viruses – took 98 per cent of its virulent cargo into the lungs. In contrast, 83 per cent of black carbon-borne viruses remained in the nasal cavity.

The scientists also observed sustained cell inflammation and damage. A blood analysis found that the particles “markedly altered biodistribution and provoked inflammatory responses to the viruses” in other organs, as well as the lungs.

The results indicate that air pollution particle-borne viruses are able to break the blood-air barrier and spread to other organs through blood circulation.

“Our results unearthed that, besides acting as a carrier, airborne fine particles could … induce detrimental effects that were different from – even more severe than – those produced by airborne fine particles alone or solo viruses,” the researchers said.