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.

    外出旅行者发生血凝块的危险几乎是正常情况的三倍，乘坐汽车或飞机每两个小时可适度增加。

    他们说这个危险严重性值得研究，用最好的方法保持旅行者健康，尽管不足以证明给旅行者抗凝药物是正当的。

    波士顿哈瓦德大学的大卫，蝉扎医生和他的同事特别地调查研究了静脉血栓栓塞--静脉凝血块的形成通常在腿上。

    当血凝块在动脉内形成时也可导致中风和心脏病，但是静脉血栓栓塞能导致局部损伤或转移到肺脏和死亡。

    蝉扎小组作了多次分析，把许多不同的研究结果汇集在一起来看他们发现的结果。

    他们找到14项研究牵涉到4000名满足这个质量标准的病人。

    我们的发现显示旅行和静脉血栓栓塞存在着明显的联系，他们在报告中写道。这篇报告出版在埃纳尔内科杂志上。

    他们发现孕妇和服用避孕药的妇女及肥胖者尤其危险。

    这种绝对的危险在每4600个航空旅行者中总会有一例。他们注释，一些研究显示，不存在血凝块的危险，而是说他们所采用的研究方法可能会被质疑。

    "这个报告的发现建议：在一般健康的个体中至少增加三倍的相对危险性，不可能产生足够绝对的危险。不能证明采取高危干预措施是正当的，像在旅行期间口服抗凝药物。"他们写道。

    但是确保人们饮用大量的液体饮料，每隔两小时站起来活动等等是有价值的，他们说。

    "在2010年，全世界25亿旅客将乘飞机旅行，世界大量人口将处在这个不可大意的条件的危险之中。"他们写道。