In addition to window seats and having already had DVT/PE or pulmonary embolism, risk factors include cancer, recent surgery, advanced age, estrogen use (including oral contraceptives), pregnancy, and obesity. The association between air travel and DVT/PE is strongest for flights longer than eight to 10 hours, and most passengers who develop the condition after long-distance travel have one or more risk factors. Immobility related to car, bus, and train travel can also increase the risk of DVT/PE.

To be fair, the news guidelines appeared three weeks after Donnelly’s article, but a few phone calls surely would have prevented him from carelessly touting “economy class syndrome” and the use of anticoagulants. Instead of quoting any chest, heart, or blood doctors, however, he relied on a couple of infectious disease experts who are highly regarded in their fields, but unqualified to speak about the risk of DVT/PE from long-distance travel. He also cited a website called, without mentioning that its “premier sponsor” is Sanofi-Aventis, which makes DVT drug Lovenox.

The new guidelines have drawn significant media coverage in the last week, and most articles have conveyed their content fairly accurately, although a few articles didn’t do a good job explaining the difference between absolute and relative risk.

The College of Chest Physicians’ guidelines found that the chance of developing DVT/PE in the month following a flight longer than four hours is one in 4,600 flights. An article on mentioned the statistic as well as the finding that “the risk rises by 18% for each two-hour increase in the duration of travel,” all of which is accurate. But the piece should have stressed that an increase in relative risk of 18 percent amounts to a very small increase in absolute risk—so the chance of developing DVT/PE is .02 percent after a four-hour flight, and only .03 percent after a six-hour flight.

Likewise, an article in USA Today reported that, “A report by the World Health Organization in 2007 found that for every four hours of immobilization, the chance of experiencing a blood clot doubles [link added],” which sounds scary, but belies the fact that, even then, the health organization said the absolute risk was only about one in 6,000.

Finally, a CBS News/Associate Press story reported that, “The average risk for a deep vein blood clot in the general population is about 1 per 1,000 each year. Long-haul travel doubles the chance, but the small risk should reassure healthy travelers that they’re unlikely to develop clots….” Again, this is true, but the piece could’ve done more to emphasize how small the risk is by explicitly stating that where the risk of DVT/PE for non-travelers is a tenth of a percent, it’s only two-tenths of a percent for those with more wanderlust.

None of these flaws was as bad Donnelly’s personal-narrative-gone-wrong, however. Frightening experiences with a medical problem can make a very effective frame in health reporting (to see it done right, check out Alice Park’s first-person coverage of the new DVT/PE guidelines in Time). But the writer must buttress his or her tale with a thorough accounting of the scientific evidence and quotes from qualified sources. Unfortunately, Donnelly did neither.

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Curtis Brainard writes on science and environment reporting. Follow him on Twitter @cbrainard.