in-out.md

First, there are [3 science based methods]({{<relref "methods">}}) to clean the air:

• Ventilation - bringing in outdoor air
• Filtration - filtering particles from the air
• UV - killing microbes with UV light

These can be done in-room and out-of-room (in the HVAC unit).

In-Room Mitigation

Ventilation:

• increase air quantity entering the room
• open doors/windows
• turn on local exhaust fans

Filtration:

• HEPA Filters
• CR Boxes

UV:

• Upper room UV
• Stand alone UV

Out-Of-Room Mitigation (in HVAC system)

Ventilation:

• increase the outdoor air % in the HVAC unit

Filtration:

• upgrade the filter in the HVAC unit from MERV-8 to MERV-13

UV:

• install UV lights in the HVAC unit, either by the coils or in the supply duct

To illustrate from @DavidElfstrom's picture:

To illustrate from @CPHO_Canada's picture:

Why is In-Room mitigation better than Out-Of-Room mitigation?

First we need to know how COVID spreads. #COVIDisAirborne and generally spreads in 3 situations:

1. Short Range - up close
2. Shared Room - same room for long period
3. Long range - different rooms, distanced & short interaction...

{{< tweet user="jljcolorado" id="1479251740570566658" >}}

Shared room transmission is higher risk than long range, so it should be the primary focus for mitigation. In-Room mitigation works for all types of spread (including close-range).

{{< tweet user="jljcolorado" id="1496699591558930435" >}}

All the In-Room mitigation methods are additive. They individually work to clean the air. Out-Of-Room methods clean the recirculated air. They aren't as effective:

1. All methods clean the same air. Once it's clean, more mitigation doesn't help. Diminishing returns.
2. If you are supplying air from one zone to another through a HVAC unit, that is long range transmission and lower risk (but still possible).
3. If the HVAC unit supplies many zones, then the recirculated air is diluted with air from all the zones. Virus concentrations supplied to the spaces will be low.
4. There are cases where out-of-room mitigation is completely useless, like HVAC units without recirculated air.
5. Is it possible that heating/cooling coils or going through the unit can damage the virus anyways? If the HVAC unit supplies a single space, then improvements are similar to in-room mitigation. The larger the HVAC unit is and the more zones it supplies, improvements only protect against long range transmission.

Out-Of-Room mitigation shouldn't be completely dismissed. There are other health benefits. Bringing in more outdoor air lowers CO2 levels and improves cognition and health. MERV-13 filters remove all PM2.5 which is associated with adverse health effects.

Long range transmission still happens and mitigation is worthwhile, but shared room transmission is higher risk and should have higher focus. For smaller HVAC units, it's still worthwhile to use the out-of-room methods.