What are Respirators Protecting Us Against?

Droplets
A primary reason for wearing a respirator is to protect from droplets. For example if a sick person coughs or sneezes when in close proximity to us, the respirator forms a barrier to prevent their bodily fluids reaching our face.

Droplets are generally large, and gravity drags them down to land on objects, rather than staying in the air. So they don’t travel very long distances. There is however research into micro droplets, which get ejected even during talking. This Vimeo video made by Japanese researchers, captures micro droplets on video using high speed cameras. We know large droplets play a role in transmission, but it’s not yet clear what role micro droplets play.

Aerosols
What may remain in the air for some time are aerosolized virus particles. So for example, you could imagine someone creating two issues when sneezing, the first are ejected droplets, which travel a short distance, then second, aerosolized virus particles that stay in the air for longer.

Currently there is debate and uncertainty around how long Covid-19 can remain aerosolized, and how much of a risk that vector is compared to others.

What we can do is be aware of what research currently says, and err on the side of caution until its been confirmed.

Scientists at the National Institute of Allergy and Infectious Diseases (NIAID) published a study in NEJM (link) on what can happen under Controlled Environment. They used a nebulizer, which creates an aerosol from liquids, and tested how long the virus remains measurable in the air whilst aerosolized. They also tested how long the virus was measurable on other surfaces. Their results showed the virus remained measurable for the full duration of the aerosolization experiment which is 3 hours.