Objective The objective of this study is to determine the association between the duration of high-altitude (> 3000 m) pre-exposure and acute mountain sickness (AMS) incidence. Methods A prospective observational study was conducted on 2 random days each month from April 2007 to March 2008 at Paiyun Lodge (3402 m), Jade Mountain, Taiwan. Demographic data, prior AMS history, symptoms, and scores and the days and times of high-altitude pre-exposure within the preceding 2 months were obtained from lowland (< 1500 m) trekkers. Results Totally, 1010 questionnaires were analyzed; 106, 76, and 828 trekkers had pre-exposure lasting at least 3 days (group 1), less than 3 days (group 2), and 0 days (group 3), respectively. Acute mountain sickness incidence was significantly higher in groups 2 and 3 than in group 1 (21.70%, 35.53%, 37.08%, respectively; P =.008). Logistic regression analysis indicated a significantly lower AMS risk in group 1 (group 1, P =.004; odds ratio [OR], 0.479; 95% confidence interval [CI], 0.290-0.791; group 2, P =.226; OR, 0.725; 95% CI, 0.430-1.221). In group 1, 28 and 78 trekkers had single and intermittent multiple pre-exposure, respectively. There was no difference in the incidence or severity of AMS symptoms between single and intermittent multiple pre-exposure (AMS, P =.838; headache, P =.891; dizziness or lightheadedness, P =.414; fatigue and/or weakness, P =.957; gastrointestinal symptoms, P =.257; difficulty sleeping, P =.804; AMS score, P =.796). Conclusions High-altitude pre-exposure lasting at least 3 days within the preceding 2 months was associated with a significant lower AMS incidence during a subsequent ascent among Jade Mountain trekkers.
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