Experts Urge More Attention to Airborne Viruses at Medical Facilities

The infection control policies in force at many health care facilities are based on outdated models of how respiratory viruses spread and badly need revision, according to a team of leading experts that includes a University of Maryland researcher.

Their review of current best practices and suggestions for policy updates was published yesterday in Annals of Internal Medicine.

Traditional medical teaching suggests that most respiratory viruses are spread through droplets from an infected person, and that these larger particles are heavy enough that they rapidly fall to the ground within one to two meters.

Public health agencies have traditionally advised health care workers to wear surgical masks to protect themselves from pathogens in the droplets, except in instances where patients undergo so-called “aerosol-generating procedures” such as intubation, which were thought to generate far smaller droplets known as aerosols that linger in the air. In those case, higher levels of respiratory protection, such as N95 respirators, are currently recommended.

But the review shows that we need N95 masks in a much broader set of circumstances, said study co-author Dr. Donald Milton, a professor of occupational and environmental health in the UMD School of Public Health.

“In the past, there was a false dichotomy between droplets and aerosols that underestimated the importance of aerosols,” he said. “It is time to recognize that aerosols are playing a critical role in the transmission of respiratory viruses, not just SARS Co-V2, but influenza and others.”

In addition to Milton, researchers from Harvard Medical School, Harvard Pilgrim Healthcare Institute and the University of Maryland School of Medicine in Baltimore reviewed a slew of published studies looking at SARS-CoV-2 transmission and infection control policies. Most current studies now suggest that respiratory viruses are primarily transmitted by aerosols that become concentrated close to the source patient, persist in the air and can travel beyond two meters—and most importantly, can bypass surgical masks.

People routinely generate aerosols whenever they exhale, particularly when speaking loudly, breathing heavily or coughing—and most of the designated “aerosol-generating procedures” that call for extra precautions under current policies don’t meaningfully increase aerosol generation any more than talking or heavy breathing. The risk of inhaling infectious aerosols is greatest close to an infected person and in poorly ventilated spaces, and prevention is more difficult because people can be infectious without having symptoms.

“Under current infection-control thinking, you only use respirators for personal protection when you know someone is infected,” Milton said. “The problem with SARS-CoV-2 is that you don’t know when someone is infected.”

The authors urged researchers and public health specialists to re-examine transmission-prevention methods and create a uniform set of precautions for all respiratory pathogens and high-risk interactions, rather than differentiating between viruses and procedures. The focus, they said, should be on preventing transmission among people regardless of symptoms and known infection status.

Milton said that while hospital infection control specialists have not fully accepted the argument about the centrality of aerosols, a recently launched investigation of aerosols in flu transmission with colleagues at the University of Maryland, Baltimore “will play a critical role; our work is laying the scientific foundation that is needed to better control respiratory infections.”