NBC’s Nancy Snyderman in her studio in 2009. She breached a voluntary 21-day Ebola quarantine this week. (Helayne Seidman/For The Washington Post)

In this year’s Ebola outbreak, those unfortunate enough to be exposed to the virus could take solace in one accepted fact: After 21 days with no symptoms, they were in the clear.

The Centers for Disease Control and Prevention asked airline crews to ask sick travelers if they were in Guinea, Liberia or Sierra Leone in the past 21 days. The family of Dallas Ebola victim Thomas Eric Duncan will be in quarantine for 21 days. And NBC’s Nancy Snyderman just broke her three-week quarantine.

Now, research by a Drexel University professor that appeared on PLOS Currents: Outbreaks questions whether 21 days is long enough to rule out Ebola.

While the 21 day quarantine value currently used may have arose from reasonable interpretation of early outbreak data, this work suggests a reconsideration is in order,” Charles N. Haas wrote in “On the Quarantine Period for Ebola Virus, “and that 21 days may not be sufficiently protective to public health.”

Haas looked at incubation times for the virus in previous outbreaks in Congo, Zaire and Uganda. He found that “12% of the time, an individual case will have a greater incubation time than 21 days” — and said some models indicated a 31-day quarantine might be in order. Indeed, data examined in the New England Journal of Medicine shows incubation times even longer than that.

In a telephone interview with The Washington Post, Haas said the CDC, the National Institutes of Health and the World Health Organization need to go back to the drawing board.

“I think the CDC or NIH or whoever is empowered to set policy — or WHO — needs to sit down with the data and do a real analysis of costs and benefits and reconsider the 21-day time criteria,” he said.

Haas said a disease’s incubation time can be underestimated looking at a small sample of victims. He believes Ebola’s purported 21-day incubation is based on a relatively small outbreak in Zaire in 1976 that may no longer be relevant.

“If you randomly pull 21 people from The Washington Post newsroom and measured their height, somebody would be the tallest,” he said. “But they would likely not be the tallest person in D.C.”

Though Haas did not recommend a specific quarantine period, he said officials at CDC “probably try too much to allay public concern by portraying an air of confidence.” Or, as he put it in his paper: “Twenty-one days has been regarded as the appropriate quarantine period for holding individuals potentially exposed to Ebola Virus (EV) to reduce risk of contagion. … There does not appear to be a systematic discussion of the basis for this period.”

This post has been updated.