Environmental Health Hazards International travelers may be subject to certain stresses that lower resistance to disease, such as crowding, disruption of usual eating and drinking habits, and time changes with jet lag contributing to a disturbed pattern of the sleep and wakefulness cycle. These conditions of stress can lead to nausea, indigestion, fatigue, or insomnia. Complete adaptation depends on the number of time zones crossed, but may take a week or more. Excessive heat and humidity may contribute to heat exhaustion due to salt and water deficiency; or to the more serious heat stroke or hyperthermia. The ultraviolet rays of the sun can cause severe and very debilitating sunburn in lighter-skinned persons. Excessive cold affects persons who may be inadequately dressed and particularly the elderly; it can lead to hypothermia and to frost-bite of exposed parts of the body. Breathing and swallowing dust when traveling on unpaved roads or in arid areas may be followed by nausea and malaise, and may cause increased susceptibility to infections of the upper respiratory tract. Traveling in high altitudes may lead to insomnia, headache, nausea, and altitude sickness, even in young and healthy persons, and can cause distress to those with cardiac or pulmonary conditions. Individual susceptibility to acute mountain sickness is highly variable. Travelers who are at greatest risk are those who ascend rapidly to tourist sites in the Andes and Himalayas. Medication is available to hasten the process of acclimatization to high altitudes.