No sensor can monitor how many infectious aerosols are swirling around us in real time. But carbon dioxide, or CO2, can act as a convenient proxy. People exhale it when they breathe, and in spaces that aren’t well ventilated, the gas accumulates. High CO2 concentrations can provide a warning sign that a lot of the air you’re inhaling is coming out of other people’s respiratory tracts.

For decades, that’s how aerosol scientists and ventilation engineers have mostly thought about CO2 — as a sort of indicator for the health of indoor environments. But over the last three years, researchers in the U.K. working with next-generation bioaerosol technologies have discovered that CO2 is more than a useful bystander. In fact, it plays a critical role in determining how long viruses can stay alive in the air: The more CO2 there is, the more virus-friendly the air becomes.

It’s a revelation that is already transforming the way scientists study airborne pathogens. But on a planet where burning fossil fuels and other industrial activities inject 37 billion metric tons of CO2 into the atmosphere each year, it could also have huge implications for human health.

By increasing the CO2 in the air, we’re getting rid of a natural means by which viruses become inactivated.