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By now, this is rule all of us now know by heart: We have to maintain a distance of six feet from other people so we each do our part in preventing the spread of COVID-19.

But is six feet really enough? Here’s what you need to know:

Where does the six-foot rule come from?

Health experts have said the recommendation for six feet of social distancing comes from studies of diseases in the 1930s and 1940s, mainly the work of William F. Wells, who studied tuberculosis.

COVID-19 is believed to be spread mainly through large respiratory droplets produced when people cough or sneeze.

The droplets typically travel three to six feet.

Health experts say that because the droplets are larger and can usually be seen with the naked eye, gravity will cause them to fall to the ground within that distance.

Why are some health experts concerned about the six-foot rule?

There are questions about whether the virus can be transmitted through microscopic droplets, called aerosols, which linger in the air.

A study published in the Journal of the American Medical Association in late March found that droplets that were produced when someone coughs or sneezes can travel up to 27 feet.

The study wasn’t conducted on the COVID-19 virus specifically, and droplets survive and fall at different rates depending on their size and factors such as temperature, humidity and air currents.

The study could have implications for the novel coronavirus, but there’s still a lot that isn’t known about the disease, such as how much of the virus survives in smaller particles and how much of a living virus it takes to make someone sick, health experts say.

Even if the virus lingers in respiratory droplets in the air, health experts still can’t say whether the droplets hold enough of the virus to cause concern.

What about social distancing when you exercise?

Because of the varying theories about aerosol transmission, questions have been raised about safe social distances outdoors. Some experts say the risk of transmission outside is low because air flow dilutes any air expelled air.

Generally, health experts say you’re safer engaging in outdoor activities than indoor ones, where air flow can be restricted, people are close together and frequently touched items are abundant.

Health experts say allowing more space makes sense because people breathe heavier and harder when they’re exercising.

So, is six feet enough?

Many health experts say six feet of distance is a good minimum to aim for, based on what is known about the virus’s main method of transmission. Health experts also recommend taking personal factors into account, such as how vulnerable an individual is and how well air is flowing.

Tribune News Service