Infection Risk of COVID-19 Calculator for Indoor Environments

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Probability of Infection Calculator

For Indoor Environments

Using the Wells-Riley equation, we built this simple analysis tool to help building operators understand the contribution of the HVAC system to the overall risk of spreading the virus which causes COVID-19 in buildings and the utility of different operation strategies in decreasing that risk, the top being:

  • increase the outside air fraction,
  • increase the overall ventilation recirculation rate,
  • implement additional filtering, or air purification methods.

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    How to use the calculator

    Developer Notes

    We’ve done a diligent QC of implementing the Wells-Riley equation for estimating the infection risk of COVID-19, an equation also used by ASHRAE. But it’s complicated and if you see this tool respond in a manner that is counterintuitive then let us know.

    As an alternative to traditional air conditioning systems, consider direct evaporative cooling as a means of cooling incoming air and adding hydration to the space as well. It provides cooling, fire protection, and air cleaning at only the cost of water.


    We’ve made assumptions about critical constants in the calculator that we believe are conservative but currently have a high degree of error.

    Key among them is the quanta assumption for COVID which at time of publishing has a high degree of error associated with it.

    We’ve assumed 48 as our quanta constant in this implementation. If you’d like a version of this calculator that allows you to adjust the assumption for quanta, let us know.

    Need guidance on re-opening or operating your building safely during COVID-19?