New research report

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How much virus is released into the environment?

Coming out of “lockdown” means mixing more with other people and potential COVID-19 carriers. How big is the risk?

Getting to the point, research shows that staying too long in enclosed indoor spaces with limited fresh air or with recycled air and lots of people should be avoided.

That’s the take home message of a very useful article written by Dr Erin Bromage, an Associate Professor of Biology at the University of Massachusetts Dartmouth.

In his article The Risks – Know them – Avoid them, Dr Bromage details the rate of infection across a range of environments and gives sage advice what to avoid. Here’s a summary.

Social distancing rules are really to protect you with brief exposures or outdoor exposures. In these situations, there is not enough time to achieve the infectious viral load when you are standing 6 feet apart or where wind and the infinite outdoor space for viral dilution reduces viral load. The effects of sunlight, heat, and humidity on viral survival, all serve to minimize the risk to everyone when outside.

As we start to venture out more, possibly even resuming in-office activities, you need to look at your environment and make judgments. How many people are here, how much airflow is there around me, and how long will I be in this environment. If you are in an open floorplan office, you really need to critically assess the risk (volume, people, and airflow). If you are in a job that requires face-to-face talking or even worse, yelling, you need to assess the risk.

If I am outside, and I walk past someone, I would have to be in their airstream for 5+ minutes for a chance of infection. While joggers may be releasing more virus due to deep breathing, remember the exposure time is also less due to their speed. Please do maintain physical distance, but the risk of infection in these scenarios are low. Here is a great article in Vox that discusses the low risk of running and cycling in detail.

While I have focused on respiratory exposure here, please don't forget surfaces. Those infected respiratory droplets land somewhere. Wash your hands often and stop touching your face!


Bathrooms have a lot of high touch surfaces, door handles, faucets, stall doors. So, the transfer risk in this environment can be high. We still do not know whether a person releases infectious material in feces or just fragmented virus, but we do know that toilet flushing does aerosolize many droplets.

Treat public bathrooms with extra caution (surface and air), until we know more about the risk.

Cough and sneeze

A single cough releases about 3,000 droplets and droplets travels at 50 miles per hour. Most droplets are large, and fall quickly (gravity), but many do stay in the air and can travel across a room in a few seconds.

A single sneeze releases about 30,000 droplets, with droplets traveling at up to 200 miles per hour. Most droplets are small and travel great distances (easily across a room). If a person is infected, the droplets in a single cough or sneeze may contain as many as 200,000,000 (two hundred million) virus particles which can all be dispersed into the environment around them.


A single breath releases 50 - 5000 droplets. Most of these droplets are low velocity and fall to the ground quickly. There are even fewer droplets released through nose-breathing. Importantly, due to the lack of exhalation force with a breath, viral particles from the lower respiratory areas are not expelled. Unlike sneezing and coughing which release huge amounts of viral material, the respiratory droplets released from breathing only contain low levels of virus.


Speaking increases the release of respiratory droplets about 10-fold; about 200 virus particles per minute. Again, assuming every virus is inhaled, it would take about 5 minutes of speaking face-to-face to receive the required dose.

Anyone you spend greater than 10 minutes face-to-face is potentially infected. Anyone who shares a space with you (say an office) for an extended period is potentially infected. This is also why it is critical for people who are symptomatic to stay home. Your sneezes and your coughs expel so much virus that you can infect a whole room of people.

Symptomatic people are not the only way the virus is shed. We know that at least 44% of all infections--and the majority of community-acquired transmissions--occur from people without any symptoms (asymptomatic or pre-symptomatic people). You can be shedding the virus into the environment for up to 5 days before symptoms begin.

Infectious people come in all ages, and they all shed different amounts of virus.

Keep washing your hands and don’t touch your face.

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Immunologist Erin Bromage. Photo credit: Boston Herald

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