Open a window to reduce virus spread, CDC tells schools - CNN

It's the first time the agency has separately emphasized the role ventilation plays in helping or preventing the spread of the virus, which is transmitted in tiny particles called aerosols, as well as via larger particles. The CDC had previously mentioned ventilation in its guidance to schools, but now dedicates a full page to the issue.

"If safe to do so, open windows and doors," the CDC advises. "Even just cracking open a window or door helps increase outdoor airflow, which helps reduce the potential concentration of virus particles in the air. If it gets too cold or hot, adjust the thermostat."

It's important to make sure open windows are not dangerous, the CDC noted. "Do not open windows or doors if doing so poses a safety or health risk (such as falling, exposure to extreme temperatures, or triggering asthma symptoms)," it said.

The same goes for buses.

"Open windows in transportation vehicles." The more windows that can be opened, the better, but even cracking windows helps, the CDC said.

Mask use can prevent the virus from getting into the air in the first place, the CDC said on the newly posted web page. But good ventilation is also important.

"Use child-safe fans to increase the effectiveness of open windows. Safely secure fans in a window to blow potentially contaminated air out and pull new air in through other open windows and doors," it adds. "Consider having activities, classes, or lunches outdoors when circumstances allow."

The new guidance also addresses heating, ventilation and air conditioning (HVAC) systems. "Set HVAC systems to bring in as much outdoor air as your system will safely allow," it suggests. "Increase the HVAC system's total airflow supply to occupied spaces when you can. More air flow encourages air mixing and ensures any recirculated air passes through the filter more frequently."

Filtration is also important, but filters should not reduce airflow.

Certain air filters can be useful, also, the CDC said.

"Consider portable air cleaners that use high-efficiency particulate air (HEPA) filters to enhance air cleaning wherever possible, especially in higher-risk areas such as a nurse's office or sick/isolation room," it recommended.

Likewise, properly installed ultraviolet lights -- they should be high up, at ceiling level -- may help, the CDC said.

"Consider using ultraviolet germicidal irradiation (UVGI) in schools and non-home-based child care programs as a supplemental treatment to inactivate the virus that causes COVID-19, especially if options for increasing ventilation and filtration are limited. Consult a qualified professional to help design and install any UVGI system," the CDC said.

With poor ventilation, viral particles can build up in the air in a classroom, cafeteria or hallway.

"If you look at all the high profile outbreaks -- same underlying factors -- no masks, low ventilation. It doesn't matter if it's spin class, ice hockey, camps, classrooms, choir practice or restaurants, (it's) the same underlying factors," Joseph Allen, who directs the Healthy Buildings Program at the Harvard T.H. Chan School of Public Health, told CNN earlier this month.

Ventilation experts have been urging the CDC to say more about the importance of clean air in schools.

"The state of ventilation in schools in the United States right now is woefully inadequate," Richard Corsi, dean of the College of Engineering and Computer Science at Portland State University, told a forum hosted by The Johns Hopkins Center for Health Security at the Bloomberg School of Public Health this week.

But they said schools should avoid falling for gimmicky products whose promoters claim they clean or disinfect air.

What is most important, experts agreed, is fresh air.

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