How viruses spread indoors
We’ve all heard a lot of theories lately about what’s “safe” to do indoors with other people—specifically with regard to preventing the spread of COVID-19. You may be wondering, how exactly do viruses spread indoors?
How do viruses work?
The first thing you need to know is that viruses cannot live on their own; they need a host to survive. That means that if a surface is contaminated with virus particles, the virus will only survive for so long if it’s left alone. How long viruses can live on surfaces depends on the virus and the surface, but they are generally thought to stay viable for several hours to several days.
Viruses can be spread through sneezing, coughing, exhalation or blood contact. They typically enter the body through inhalation or a break in the skin.
In the case of coronavirus, inhalation (breathing in respiratory droplets) is thought to be the main mode of transmission.
Originally, it was thought that maintaining a social distance of 6 feet could sufficiently control the spread of coronavirus. Unfortunately, recent studies have found that smaller virus droplets can travel farther and remain airborne for longer than previously thought. This increases the risk that people sharing indoor spaces could pass the virus to one another, even if they are not in an indoor space at the same time. For example, if person A exhaled coronavirus droplets in a store and left, and then person B entered the space, the contaminated virus droplets could still be lingering in the air.
How does coronavirus travel?
While viruses can’t move themselves, they can attach themselves to larger airborne particles to “hitch a ride” until they find a suitable host.
Virus particles can also linger in the air by themselves for an indefinite amount of time.
How to control the spread of viruses
Coronavirus is primarily spread by respiratory droplets, so controlling air particulates is a primary concern. So what determines how long aerosol droplets (airborne particles) are a threat? And how can we control them?
Particle Sizes
You may remember that we talked some time ago about particle sizes. Individual viruses are some of the smallest particles around. They measure .005 to .3 microns (compared to dust, which ranges from 3 to 100 microns). That’s so tiny that you’d have to use a microscope to see them.
Small particles like this usually pose the biggest health risks. Particles that are 2.5 microns and smaller are tiny enough that they can bypass all your body’s physical barriers when you breathe them in. That means they go straight to the bottom of your lungs where they can enter the bloodstream. Smaller particles can also stay in the air longer than larger particles, which increases the risk of transmission.
Filtration
Because viruses are so small, the can be difficult to remove from indoor air circulation. However, High-Efficiency Particulate Air (HEPA) filters can be an effective means of eliminating these pathogens from high-traffic occupant areas. HEPA filters need a MERV (minimum efficiency reporting value) rating of 13 to 16 can effectively remove some virus particles from the air, but some spaces may require surgical-suite level MERV filters (rated 16-20) to be effective.
Ventilation
We’ve all heard that coronavirus is less likely to spread outdoors than indoors. That’s because outdoor air typically has excellent ventilation. However, people spend at least 90% of their time indoors these days. Particulate concentrations indoors are usually two to five times higher indoors then outdoors, so the key is to provide ventilation adequate to cycle out those particulates.
The gold standard for ventilation is “passive house”, where the ventilation is 100 percent fresh air (in other words, no air is recirculated. The CDC and ASHRAE have recognized this type of ventilation as a potential method for reducing coronavirus transmission. Because no air is recirculated, coronavirus particulates that might linger in the air for an indefinite time period are removed.
Humidity
Introducing more outdoor air will allow greater ventilation, but may also increase humidity above preferred levels (40 to 60 percent relative indoor humidity). Also keep in mind that viruses love dry air (because they can travel farther). The secret to ideal humidity is this: if ventilation is modified, humidity should be reevaluated, too.
How effective are these controls?
While research on coronavirus is still relatively new, it does appear that ventilation and other buildings controls could majorly reduce aerosol transmission of viruses. For example, in one study that focused on droplet and aerosol transmission of influenza in a high school, the findings suggested that increasing ventilation to recommended levels could have the same efficacy as vaccinating 50 to 60 percent of the population.
A final word
In a world where we have little control, improving ventilation of indoor air is a relatively easy and cost-effective solution for reducing virus transmission and protecting building occupants.
