Thread by @joeyfox85

How long until a room is safe?

You enter a room that hasn't had people there for four hours. Is masking needed? Do you need to flush the space (windows+HEPA)? How long does it need to be? Here's how you calculate it.

1/12

TLDR - The only people to weigh in on this is @O_S_P_E Indoor Air Quality Advisory Group which recommends 6 hours. I think it's a little conservative and you can go down to 4. Here's the math.

2/12
ospe.on.ca/indoor-air-qua…

Disclaimer: "Safe" is the wrong word. The question is when does the room become reasonably low risk.

We'll start with flushing spaces. Once people leave, you can leave the ventilation on to clear out the space so when others enter risk is low. How much do you need to flush?
3/12

ASHRAE has weighed in on this. They recommend 3 air changes = 95% reduction in virus concentration. If you want to be very conservative, 5 air changes = 99% reduction. @O_S_P_E recommends 3 air changes too.
4/12

ashrae.org/file%20library…

You don't need mechanical ventilation to give you 3 air changes. You get them naturally. There are 3 mechanisms:
1. Infiltration (I): air uncontrollably entering the building through the envelope or openings.
2. Decay (DC): virus naturally inactivating over time

5/12

3. Deposition (DP): virus landing on surfaces, so you can't inhale it.
So find the values for these and I+DC+DP = natural air change rate.
Here are the values:

6/12

Infiltration

This depends on the building envelope and weather. These are the values from RETScreen. Most conservative is 0.2, but that would correspond to a well performing building. Most would be much higher - probably closer to 0.5 air changes per hour (ACH).

7/12

Decay

This depends on the variant, temperature & humidity. If half-life is 2 hours, then 63% decay = 1 ACH is in about 2.5 hours, so decay is about 0.4 ACH.

8/12

dhs.gov/science-and-te…

My desktop twitter froze and deleted the rest of my thread. I can’t link to the publications nicely through my phone. Once it’s working again, I’ll link to the publications.
Other publications indicate decay is 0.2-0.63 ACH. I’ll use 0.4 ACH
9/12

For deposition, literature indicates between 0.2 to 0.43 ACH. I’ll use 0.2.

So I=0.5, DC=0.4, DP=0.2 ACH

0.5+0.40+0.2 ACH = 1.1 natural ACH
So after 3 hours, you already have more than 3 air changes. Wait for four hours and you built in a safety buffer.
10/12

@O_S_P_E guidance assumes worst case scenario. To be more conservative, wait for more air changes or use lower rates for infiltration, deposition or decay. It’s 1 in a million odds at that point.

11/12

For flushing rooms yourself with windows and HEPA, see this:
12/12

Deposition rates 0.2-2 ACH line 357
Decay rates 0.2-0.6 ACH line 330
See calculator here:

This publication uses a decay rate of 0.63 ACH and deposition rate of 0.43 ACH

sciencedirect.com/science/articl…