Let’s talk about day hiking and altitude sickness, otherwise known as acute mountain sickness (AMS).
Here in Southern California we have something called the Six-Pack of Peaks Challenge. Hikers register at the SPoP’s website, summit 6 different peaks included on the challenge at their own pace, then get invited to a completion party.
Sounds good, but for the casual, So Cal day hiker, some of those peaks are rather tall. Like Mt. Baldy, 10,000 feet above sea level tall. Hikers in the area, living near sea level, but wanting to conquer Baldy, and five of his friends are at risk for altitude sickness.
This article will focus on this style of day hiking as it relates to altitude sickness/AMS. If you’re looking for advise on Mt. Everest, you’ve come to the wrong shop for that.
What is Altitude Sickness?
Altitude sickness is part of a set of altitude illnesses that center around the body’s reaction to the air having a lower density, or lower amount, of oxygen in it at altitude. Altitude sickness is a colloquial term that usually refers to Acute Mountain Sickness.
Aside from being at altitude, the diagnosis for Acute Mountain Sickness, using something called the Lake Louise Consensus Criteria, requires that you have a headache plus one more symptom that is any of the following: gastrointestinal (loss of appetite, nausea, vomiting), or constitutional (dizziness, lightheadedness, fatigue, weakness, or feeling of lack of energy). You may also feel as though you have an alcohol based hangover, with a pounding headache in the front of your head, or like you’re getting sick, but without a fever or muscle aches.
Another notable feature is water retention, where your urine output is reduced. However, as your body adjusts to the new altitude, and your mountain sickness symptoms resolve, you may have the opposite effect and experience increased urine output.
Further, acute mountain sickness is a mild form of another altitude illness called High-Altitude Cerebral Edema (HACE). You can learn more about this condition below, after we conclude our discussion on hiking and altitude sickness. But basically as your body is retaining more fluid, it coincidentally is depositing more fluid in and around the brain via leaky blood vessel capillaries, increasing the pressure in and around it.
At what altitude can you get acute mountain sickness?
You can get acute mountain sickness as altitudes starting as low as 6,500 feet (5,000 – 7000 foot range), but it becomes more common at altitudes over 8,000 feet. And it becomes even more common at altitudes over 10,000 feet. I’ll put up the quantified risks below in the tables.
Sleeping overnight at altitude increases your risk
You’re more likely to get altitude sickness if sleeping at the altitude you ascended to, because while you’re asleep, you’re not breathing in as much oxygen, and the pressure inside the blood vessels in your head is higher when lying down. This higher pressure, alongside the increased blood flow to the brain discussed below, allows the blood vessel capillaries there to leak fluid, which contributes to your altitude sickness.
This is helpful to know for day hikers, because their risks would be rationally lower than some of the numbers on the table below, since some of them are based on people sleeping at the quoted altitudes.
Quantified risks of Acute Mountain Sickness at different altitudes
| altitude | description | chances of AMS | reference |
|---|---|---|---|
| 6,500 ft (1,980 m) | “Some people can develop AMS as low as 6,500 ft….” | unspecified; likely around 10% or 1 in 10 people, based on MedScape reference below | Patient education: High-altitude illness (including mountain sickness) (Beyond the Basics); Scott A Gallagher, MD, Peter Hackett, MD |
| 8,000 ft (2,400 m) | “…it occurs in approximately 25 percent of those sleeping above 8000 feet (but who normally live at low altitude)” | 25%, or 1 in 4 people sleeping overnight | Patient education: High-altitude illness (including mountain sickness) (Beyond the Basics); Scott A Gallagher, MD, Peter Hackett, MD |
| 8,200 ft (2,500 m) | “10-25% of all unacclimated persons at 2500m or higher” | 10-25%, or 1 in 10 to 1 in 4 people | Acute Mountain Sickness; Ryland Pace, MD |
| 10,000 ft (3,000 m) | “…it occurs in approximately 40-50 percent of those sleeping above 10,000 feet (but who normally live at low altitude)” | 40-50%, or 1 in 2 to 2.5 people sleeping overnight | Patient education: High-altitude illness (including mountain sickness) (Beyond the Basics); Scott A Gallagher, MD, Peter Hackett, MD |
| 6,500-11,500 ft (2,000-3,500 m) | “In moderate altitude (2,000-3,500 m) US ski resorts, the incidence ranges from 10-40%” | 10-40%, or 1 in 10 to 1 in 2.5 people | Altitude Illness – Cerebral Syndromes on MedScape (sign up for account to view); N Stuart Harris, MD, MFA, FACEP |
| 12,500-13,300 ft (3,800-4000 m) | “Travelers flying to a high altitude destination such as Lhasa, Tibet (3,810 m; 12,500 ft) or La Paz, Bolivia (4,062 m; 13,327 ft) can expect an AMS incidence of 25-35%” | 25-35%, or 1 in 4 to 1 in 3 people (international travelers flying in) | Altitude Illness – Cerebral Syndromes on MedScape (sign up for account to view); N Stuart Harris, MD, MFA, FACEP |
| 13,000 ft (4,000 m) | “Rapid ascent to approximately 4,000 m (continental US) has been associated with incidences of 60-70%” | 60-70%, or 6 in 10 to 7 in 10 people rapidly ascending | Altitude Illness – Cerebral Syndromes on MedScape (sign up for account to view); N Stuart Harris, MD, MFA, FACEP |
| 13,000 ft (4,000 m) | “In those who hike above 4,000 m (international travel)(and so ascend at a moderate pace), 25-50% will suffer from AMS” | 25-50%, or 1 in 4 to 1 in 2 people hiking at a moderate pace outside US | Altitude Illness – Cerebral Syndromes on MedScape (sign up for account to view); N Stuart Harris, MD, MFA, FACEP |
| 14,700-18,000 ft (4,500-5,500 m) | “50-85% of (all unacclimated) people at 4500-5500 meters” | 50-85% or 1 in 2 to 8.5 in 10 people | Acute Mountain Sickness; Ryland Pace, MD |
Implied risk curve, based on data above
Based on the data above, I built this acute mountain sickness risk curve to smooth out all the various conditions found in the references’ respective studies. Single altitudes that had a range of probabilities were assigned the average of those. Altitude ranges paired with probability ranges in the data were held as such on the graph, so that the lower altitude and lower probability were coupled and visa versa.
Altitude sickness incidence as a function of altitude in feet
0.00000247 * x^1.76 = risk of altitude sickness, based on the ‘best fit’ line I constructed above, where x is your altitude in feet. Type in your altitude on your scientific calculator (iPhone held sideways), hit the power function button (x with a little y in its upper right hand corner) and type 1.76 then hit the equals sign. Multiply that number by 0.00000247 to get the percentage chance of getting altitude sickness for that altitude based on my equation.
Time at altitude to onset of altitude sickness
The symptoms of altitude sickness usually develop within 6-12 hours of arriving at an altitude above 8,000 feet (2,400 meters). However, their onset of can vary from person to person, and may depend on factors such as the rate of ascent, the altitude reached, and the individual’s susceptibility. In some cases, acute mountain sickness may develop more quickly, within an hour of arriving at high altitude. In other cases, it may take longer for symptoms to appear – up to 48 hours – especially if a person is acclimatizing to the altitude gradually.
| time to onset of altitude sickness | reference |
|---|---|
| “AMS occurs in nonacclimatized individuals within the first 48 h after ascent to altitudes above 2,500 meters (~8,000 feet), especially after rapid ascent (1 day or less). Symptoms usually begin a few hours after arrival at the new altitude but may arise as much as a day later, often after the first night’s sleep.” | Altitude Illness – Cerebral Syndromes on MedScape (sign up for account to view); N Stuart Harris, MD, MFA, FACEP |
| “Onset usually 6-12 hours at >2500 meters (~8,000 feet)” | Acute Mountain Sickness; Ryland Pace, MD |
| “Symptoms usually occur within 6 to 12 hours of arrival at altitudes above 8000 feet (2400 m). Symptoms can begin as soon as one hour or as long as 24 hours after arriving.” | Patient education: High-altitude illness (including mountain sickness) (Beyond the Basics); Scott A Gallagher, MD, Peter Hackett, MD |
Day hiking altitude sickness – understanding the lag to onset of symptoms
The lag between when you hike up to significant altitude, and the onset of altitude sickness indicates that its symptoms are not all necessarily the direct effect of your body’s cells having less oxygen molecules to work with. More likely they’re due to your body’s physiological changes in response to this lower oxygen environment.
This includes an increase in heart rate, and amount of blood pumped by the heart per beat, a mild increase in blood pressure, fluid retention, and subsequent increased blood flow to your brain. This increased cerebral blood flow then increases the leakiness of the blood vessel capillaries in your head, allowing fluid to accumulate there, increasing the pressure in and around your brain.
Though the increased blood flow to your brain is an immediate response to less oxygen in your environment, the build up of fluid around your brain cells from leaky blood vessels takes more time. Hence if you’re only temporarily hiking to say 10,000 feet at Mount Baldy, CA, then returning back to lower altitudes on a day hike, you may not have a chance to develop some of the secondary symptoms past that higher cerebral blood flow induced headache, and the lack of energy and poorer cognitive performance from lack of O2.
Watch Miranda in the Wild attempt to summit the 14,000 ft Mt. Bierstadt, which is on the Colorado Rockies Six-Pack of Peaks ‘Extra Spicy’ edition. She is clearly suffering from the blood flow induced headache as she gets near the top.
GI side effects are more likely to lag relative to others and indicate moderate acute mountain sickness
It’s probable that the altitude sickness GI side effects, like nausea and vomiting, indicate a higher cranial pressure from this extra fluid in and around your brain, similar to how a concussion causes these effects. It’s these type effects that would likely be more delayed, and you can probably avoid them when just day hiking to a summit for a few hours.
Once you start to get nausea and possibly vomiting from your altitude, you no longer have mild AMS, but indeed have moderate AMS. You may also have coordination problems at that point. If you’re day hiking and experience moderate AMS, you should make the decision to head back down the trail to a lower altitude as soon as possible. Ascending to a higher altitude at that point will put you at higher risk of severe AMS and High Altitude Cerebral Edema.
High Altitude Cerebral Edema
High Altitude Cerebral Edema is the more extreme version of altitude sickness related fluid accumulation in and around the brain. This is a medical emergency because the amount of fluid and pressure around the brain can cause brain tissue to move out of its normal resting place. We’re talking things like part of your brain getting pushed through the hole in your skull where your spinal cord starts. Not really something you live through.
Day hiking altitude sickness that evolves into full on High Altitude Cerebral Edema (HACE) is not much of a concern for healthy regular hikers just summiting a peak, like one of those in the Six-Pack of Peaks challenge, for a few hours, then going back down the mountain and heading home for the day.
Chances of getting High Altitude Cerebral Edema
HACE is estimated to occur in about 1% or less of persons traveling above 4,000 m (13,000 feet) and in 1-3% of those with AMS.
Altitude Illness – Cerebral Syndromes on MedScape (sign up for account to view); N Stuart Harris, MD, MFA, FACEP
So when you are summiting Mount Baldy at 10,000 feet on a day hike, you’re chance of getting altitude sickness is a little above 25%, based on my crunching of the data in the last section. If only 3% of that subset of hikers developing AMS go on to develop HACE, then the overall probability of getting HACE on the Baldy hike is 0.75%, or 1 in 133 hikers. And even that hiker could probably avoid HACE by recognizing their altitude sickness and turning back to their car, instead of continuing to climb higher.
But generally it takes 1 to 3 days for you to develop HACE, so again, it’s not much of a concern for the healthy day hiker.
Further reading
If you enjoyed this article on day hiking and altitude sickness, stay tuned as I’ll be adding more altitude related illness topics as time allows. For now, just check out my front page for general camping and hiking topics related to Southern California.