Colorado, with its stunning peaks and high-altitude cities, attracts millions of visitors annually who often travel rapidly from sea level to elevations where the air is noticeably thinner. Denver, the “Mile High City,” sits at 5,280 feet, but many popular mountain destinations like Breckenridge and Vail exceed 8,000 feet. This elevation is the threshold where the body begins to experience significant physiological stress. A rapid change creates a temporary oxygen deficit, prompting the body to begin adjustment known as acclimatization. Understanding this timeline is the first step toward a safe and enjoyable experience in the mountains, as it involves distinct phases of biological change that allow the body to function efficiently in the reduced-oxygen environment.
Understanding Acute Mountain Sickness
The underlying cause of altitude-related illness is hypobaric hypoxia, the reduction in the partial pressure of oxygen in the atmosphere as elevation increases. Although the air still contains 21% oxygen, the lower barometric pressure means the total number of oxygen molecules inhaled is significantly reduced. This lack of available oxygen triggers symptoms collectively known as Acute Mountain Sickness (AMS), the mildest and most common form of altitude illness.
Symptoms of AMS typically manifest within 6 to 12 hours of arrival above 8,000 feet, though individual susceptibility varies greatly. Mild indicators often resemble a hangover, including a persistent headache, fatigue, loss of appetite, nausea, or dizziness. Mild AMS is usually self-limiting; the symptoms resolve once the body initiates its initial compensatory changes and adjusts to the reduced oxygen availability.
The Timeline of Altitude Acclimatization
Acclimatization is a multi-stage process that begins immediately upon ascent and involves a series of physiological responses aimed at restoring oxygen balance. The initial, short-term adjustment is focused primarily on correcting the immediate drop in oxygen saturation. Within the first 24 to 72 hours, the body’s most immediate response is hyperventilation, where the breathing rate and depth increase significantly to draw in more air.
This accelerated breathing helps to raise the body’s oxygen levels but has a side effect of exhaling too much carbon dioxide, leading to a temporary state of respiratory alkalosis. Mild symptoms of AMS often peak during the first night at a new elevation as the body struggles with this initial chemical imbalance. The improvement of these symptoms over the first one to three days signals the successful completion of the initial adjustment phase.
The intermediate adjustment occurs roughly between three and seven days at a stable altitude. During this phase, the kidneys increase the excretion of bicarbonate into the urine to correct the respiratory alkalosis. This process, known as bicarbonate diuresis, helps to rebalance the blood’s pH, which allows the respiratory system to maintain a higher ventilation rate. The stabilization of fluid balance and blood chemistry during this period is why many visitors feel comfortable enough for moderate activity after about four to five days.
Full acclimatization, involving long-term changes, takes weeks to months and is rarely achieved by short-term visitors. The most significant long-term adaptation is the hormonal stimulation of the bone marrow to produce more red blood cells (erythropoiesis). This increase in oxygen-carrying capacity is a slow process. Short-term visitors achieve sufficient adjustment for normal tourist activities within the first few days.
Strategies for Successful Altitude Adjustment
The most effective strategy for managing the acclimatization timeline is to follow a gradual ascent profile.
- Gradual Ascent: For destinations above 8,000 feet, spend at least one night at an intermediate elevation, such as Denver. Above 10,000 feet, avoid increasing the sleeping elevation by more than 1,000 to 1,500 feet per day.
- Climb High, Sleep Low: Spend time at a higher elevation during the day but descend to a lower altitude to sleep. Resting at a lower elevation allows the body to recover more effectively. This practice is often employed when visiting Colorado’s 14,000-foot peaks.
- Maintain Hydration and Diet: Adequate hydration is important because the dry air and increased breathing rate cause rapid moisture loss. Drink nearly twice the normal amount of water and avoid diuretics like excessive caffeine and alcohol. Consuming complex carbohydrates provides a more efficient fuel source, as the body uses oxygen more effectively to metabolize them than fats.
- Limit Strenuous Activity: Limit strenuous physical activity for the first 24 to 48 hours. This allows the body to focus its resources on respiratory and chemical adjustments rather than muscle exertion.
- Consider Medication: For individuals with a history of altitude illness or those planning a rapid ascent, prescription medication can be preventative. Acetazolamide (Diamox) can be started a day or two before arrival and speeds up the natural acclimatization process by stimulating the kidneys to excrete bicarbonate.
Recognizing Severe Altitude Illnesses
While most visitors only experience mild, temporary AMS, two rare but life-threatening conditions represent the progression of severe altitude sickness. High Altitude Cerebral Edema (HACE) occurs when fluid builds up in the brain, requiring immediate medical attention. Symptoms of HACE include a severe headache unresponsive to medication, confusion, lethargy, and a loss of coordination (ataxia), often tested by the inability to walk a straight line.
The other severe form is High Altitude Pulmonary Edema (HAPE), which involves fluid accumulation in the lungs, making breathing extremely difficult. HAPE symptoms include severe shortness of breath at rest, a persistent, wet cough, and chest tightness. Both HACE and HAPE can rapidly progress within hours. The only definitive treatment is immediate descent to a lower elevation.
If symptoms are suspected, the person must be moved down the mountain as quickly and safely as possible until medical help can be reached. Recognizing the signs and acting immediately is crucial.