September marks six months since the Philadelphia region went on lockdown due to the COVID-19 pandemic. This novel, contagious and deadly virus is even more dangerous than influenza, and in the absence of a vaccine or proven therapies to combat it, the war against COVID-19 largely rests on non-pharmaceutical interventions.

Major public-health organizations such as the CDC and WHO have published guidelines for COVID-19 regarding social-distancing measures and hygienic practices. Many recommendations are broad — such as the closure of schools and nonessential businesses. Other guidelines offer specific, quantifiable rules for interpersonal distancing (six feet), hand-washing (20 seconds), group gatherings (10-person limit), and quarantine for exposed people (14 days). These explicit rules raise the questions: What is the scientific evidence for such precise recommendations? And do these interventions actually work?

How safe is six feet?

Public-health organizations have different specific recommendations for interpersonal distancing. While the widely publicized recommendation of the CDC is six feet, the WHO recommends only one meter (a tad over three feet). The science behind these guidelines dates to experiments more than a century ago using plates spaced out on the ground to measure the distance traveled by respiratory droplets. It was concluded that larger droplets traveled only short distances and that contagion could likely spread to others within only a few feet of the “infector.”

Contemporary studies question whether six feet of separation is sufficient. Air samples taken six feet away from influenza patients contain enough concentration of viral particles to be infectious. Indoor spaces such as offices and stores often have air currents that can more widely circulate particles that remain airborne for extended periods.

Recent experiments have demonstrated that coughing and sneezing create droplet clouds that travel as far as 27 feet. These observations corroborate findings from COVID-19 hospital wards where viral air samples were detected up to 13 feet from infected patients. Therefore, although six feet may be practical, there is no reassurance that even this distance is safe. Bottom line: The farther apart, the better. For most people, close contact isn’t completely avoidable during day-to-day activities, and this is why mask wearing is an important complement to social distancing.

How many people can safely gather together at one time?

President Donald Trump’s coronavirus guidelines issued in March recommended to “avoid social gatherings in groups of more than 10 people.” Locally, the Philadelphia Department of Public Health on July 14 declared a moratorium on public gatherings of 50 or more people through February 2021, thus eliminating major in-person events such as the Mummers Parade, concerts, and spectator sports.

Avoiding large crowds is important because it is difficult to maintain social distancing in such situations. The probability that one or more attendees in a group is infected is dependent on two factors: the size of the crowd and the prevalence of the disease. Because the disease prevalence in the United States is still unknown, selecting any number at which to cap group gatherings is arbitrary. Although there is no definable cap number, the important take-home point is that the potential for transmission will be directly proportional to the number of people in a gathering, and thus bigger is not better.

Is 14 days enough of a quarantine for exposed people?

Health organizations uniformly recommend that people who have been exposed to COVID-19 self-quarantine for 14 days. The appropriate duration of quarantine is determined by the virus’ incubation period. For COVID-19, the average incubation period is about five days, with about 99% of people who develop symptoms having them within two weeks after exposure. Thus a 14-day quarantine period is appropriate.

Does hand-washing actually prevent COVID-19?

All health organizations recommend frequent hand-washing with soap for 20 seconds or more. Studies indicate that hand-washing lasts, on average, 10 seconds or less, but research has shown that this brief duration is insufficient to remove soil and germs. Washing with a lather time of at least 20 seconds has been shown to significantly eradicate more microbes. A popular tip is to sing “Happy Birthday” twice while lathering. An alternative for Philadelphia sports fans is to sing a verse of the Eagles Fight Song — a Philly Special!

Many scientists view the evidence supporting social interventions as soft because these measures have not been verified by randomized trials. But the recent widespread resurgence of cases from slackened social precautions as the country began to reopen has emphatically reconfirmed the importance of these public-health measures.

Michael Savage is the Ralph J. Roberts Professor of Cardiology and David Fischman is Professor of Medicine at the Sidney Kimmel Medical College of Thomas Jefferson University in Philadelphia.