People who travel have nearly triple the normal risk of developing a dangerous blood clot, with a measurable increase for every two hours spent sitting in a car or wedged into an airline seat.

    They said the risk is serious enough to merit research into better ways to keep travelers healthy, although not severe enough to justify giving airline passengers anti-clotting drugs.

    Dr. Divay Chandra and colleagues at Harvard University in Boston looked specifically at venous thromboembolism -- the development of a blood clot in a vein, usually in the legs.

    Blood clots can also cause strokes and heart attacks when they form in arteries but VTEs can cause local damage or travel to the lung and kill.

    Chandra's team did what is called a meta-analysis, pooling the results of many different studies to see what they found collectively.

    They found 14 studies involving 4000 patients that met their criteria for quality.

    "Our findings demonstrate for the first time a clear association between travel and VTE," they wrote in their report, published in the Annals of Internal Medicine.

    Women who are pregnant or take birth control pills and the obese have an especially high risk, they found.

    The absolute risk is one case in every 4600 airline trips, they said. They noted that some studies have shown no risk of blood clots but said the way those studies were done could be questioned.

    "The findings of this report suggest that, at least among generally healthy individuals, even a three-fold increase in relative risk is unlikely to produce a sufficiently high absolute risk to justify higher-risk interventions, such as oral anticoagulation during travel," they wrote.

    But ensuring that people drink extra fluids and get up and move every two hours or so is worthwhile, they said.

    "Worldwide, 2.5 billion passengers will travel by air alone in 2010, which underscores the large global population at risk for this serious condition," they wrote.

    外出旅行者發生血凝塊的危險幾乎是正常情況的三倍，乘坐汽車或飛機每兩個小時可適度增加。

    他們說這個危險嚴重性值得研究，用最好的方法保持旅行者健康，盡管不足以證明給旅行者抗凝藥物是正當的。

    波士頓哈瓦德大學的大衛，蟬扎醫生和他的同事特別地調查研究了靜脈血栓栓塞--靜脈凝血塊的形成通常在腿上。

    當血凝塊在動脈內形成時也可導致中風和心臟病，但是靜脈血栓栓塞能導致局部損傷或轉移到肺臟和死亡。

    蟬扎小組作了多次分析，把許多不同的研究結果匯集在一起來看他們發現的結果。

    他們找到14項研究牽涉到4000名滿足這個質量標準的病人。

    我們的發現顯示旅行和靜脈血栓栓塞存在著明顯的聯系，他們在報告中寫道。這篇報告出版在埃納爾內科雜志上。

    他們發現孕婦和服用避孕藥的婦女及肥胖者尤其危險。

    這種絕對的危險在每4600個航空旅行者中總會有一例。他們注釋，一些研究顯示，不存在血凝塊的危險，而是說他們所采用的研究方法可能會被質疑。

    "這個報告的發現建議：在一般健康的個體中至少增加三倍的相對危險性，不可能產生足夠絕對的危險。不能證明采取高危干預措施是正當的，像在旅行期間口服抗凝藥物。"他們寫道。

    但是確保人們飲用大量的液體飲料，每隔兩小時站起來活動等等是有價值的，他們說。

    "在2010年，全世界25億旅客將乘飛機旅行，世界大量人口將處在這個不可大意的條件的危險之中。"他們寫道。