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Does our body see DA?

bret

bret

Well Known Member
Does Density Altitude have the same affect on our lungs ability to absorb O2 as our far as what our planes sees in engine performance and lift? Summer flying here in the sierras we will often see 2,000 or more higher DA, so if crossing at 11,500 and DA is 13,500 what altitude do our lungs see? Not looking for regs and suggestions, Just a hypothetical question.
 
This came up before due to a misunderstanding I had. The pressure our lungs see isn't affected by density altitude. This surprised me but was explained well in another thread I can not find. I would try to explain it but am afraid I would fail!
 
It was 1980, but I didn't sleep through that class...

(Partial pressure of oxygen in the atmosphere) x (total atmospheric pressure where you are at the moment)= what you get delivered to the alveoli. If density altitude = best measure of true cabin air pressure, then DAlt absolutey affects oxygen delivery to the tissues. I see no way around it.

What were the contrary arguments made?
 
Bill Boyd said:
(Partial pressure of oxygen in the atmosphere) x (total atmospheric pressure where you are at the moment)= what you get delivered to the alveoli. If density altitude = best measure of true cabin air pressure, then DAlt absolutey affects oxygen delivery to the tissues. I see no way around it.

What were the contrary arguments made?
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Ahhh but density altitude is NOT a measure of pressure, it is an indication of DENSITY. Thinner or thicker air effects things like lift and engine power, but does NOT affect the pressure your lungs see. Yes, there are slightly fewer o2 molecules per given volume in warmer air, but the body already doesn't need many of the molecules, it needs the PRESSURE.

Also remember that the air our lungs sees is usually warmed up to almost body temperature by the time our lungs see it, no matter the altitude.

Chris
 
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Oooo......so it's pressure not density........wait, what?.....if the air is thinner, isn't lighter, and if its lighter there is less pressure???
 
Somewhere I recall a thread where it was determined that your lungs are affected by pressure altitude. Foreflight gives me an alert at 12,500' pressure altitude.
 
snopercod said:
Somewhere I recall a thread where it was determined that your lungs are affected by pressure altitude. Foreflight gives me an alert at 12,500' pressure altitude.
Click to expand...

This is correct. Your lungs convert ambient air to ~98f and 100% humidity. So pressure altitude is the only uncontrolled factor.
 
bret said:
Oooo......so it's pressure not density........wait, what?.....if the air is thinner, isn't lighter, and if its lighter there is less pressure???
Click to expand...

The pressure comes from the weight of the atmosphere ABOVE, as the air at any given altitude isn't really in an enclosed, constant volume space. If it was, Gay-Lussac's law would apply directly and hotter air would actually result in increased pressure.

In this case, the pressure is coming from the weight above the altitude being discussed, and is represented by Pressure Altitude - this is already affected by the density of the air above; ie at 5000' MSL, a low barometer reading would indicate lower than normal weight of atmosphere above, and thus a higher pressure altitude (say 7000').

I'm sure there is a better way to explain it but I'm looking over my flight physiology textbook trying to find a better example.

Chris
 
Pressure altitude

Pressure altitude as stated by others.

Moisture does play a role at altitudes above 28,000ft where the pressures are low enough that water vapor displaces oxygen to a point that we need to deliver oxygen under pressure to maintain a pO2 high enough to oxygen the blood.

But when your thinking about your own personal supplemental oxygen need and when and how you use supplemental O2 think in terms of pressure altitude.

Bugsy, 2015 president aerospace physiology society
 
snopercod said:
Somewhere I recall a thread where it was determined that your lungs are affected by pressure altitude. Foreflight gives me an alert at 12,500' pressure altitude.
Click to expand...

But may that be driven more by FAA altitude and O2 regulations?
 
Bugsy said:
Pressure altitude as stated by others.

Moisture does play a role at altitudes above 28,000ft where the pressures are low enough that water vapor displaces oxygen to a point that we need to deliver oxygen under pressure to maintain a pO2 high enough to oxygen the blood.

But when your thinking about your own personal supplemental oxygen need and when and how you use supplemental O2 think in terms of pressure altitude.

Bugsy, 2015 president aerospace physiology society
Click to expand...

I'll humbly defer to Bugsy as the expert here.

I admit to needing a better grasp of atmospheric science and the correlation between pressure altitude and DAlt. But yes, density isn't the issue, absolute pressure is (for a given FiO2). And a host of other factors, such as your altitude adaptation, COHgb, alveolar permeability, anything that shifts the ol' dissociation curve, and probably your caffeine level ;)

And in the end, as Gil said, you have to obey the regs regardless of physiology.

Thanks for weighing in, Bugsy. Fascinating topic, and shows why I'm not a pulmonologist or a flight surgeon.
 
a final thought

Given the factors that go into calculating density altitude, I'm betting pressure altitude is _the_ majority contributor. A high DAlt day is likely a high pressure altitude day, too, even with nonstandard temperature and humidity in play. Right?

Doesn't take long to dial in 29.92" and see what it reads if you're curious.
 
As mentioned your lungs ability to capture O2 is based upon the atmospheric pressure your exposed to. Your lungs have some excess capacity and can therefore tolerate lower Atmo pressures to a point without reduced O2 saturation. Beyond that point, further reduction in Atmo pressure results in lowered O2 saturation. Presumably, the FAA came up with 12,500' as a guideline based upon typical O2 sat's for the average joe at that altitude and what they considered a safe, minimum O2 sat level.

The beauty of Pressure altitude is that it wills always show a given altitude (let's say 12,500') at the same atmospheric pressure level regardless of temp, barometric pressure, humidity, etc. This is unrelated, of course, to your actual altitude. Therefore it is the best tool for creating and identifying a critical point in O2 saturation requirements, because it is reflection of actual atmospheric pressure at that moment. It is infinitely repeatable.

Larry
 
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Bill Boyd said:
Given the factors that go into calculating density altitude, I'm betting pressure altitude is _the_ majority contributor. A high DAlt day is likely a high pressure altitude day, too, even with nonstandard temperature and humidity in play. Right?
Click to expand...

No. Variations in pressure ALT (vs indicated ALT) are a small effect in calculating density altitude. Non standard temperatures are the biggest effect. Just make up some examples and calculate DA on your e6b to verify.
Edit: Just to clarify, I am talking about how much density altitude varies from indicated altitude. I believe that is common useage, e.g., if someone at KTVL (elevation 6000') says 'DA is high today", they mean in comparison to a standard day at KTVL. In an absolute sense, the DA at KTVL will always be much higher than at a sea level airport.
 
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lr172 said:
. . .Presumably, the FAA came up with 12,500' as a guideline based upon typical O2 sat's for the average joe at that altitude and what they considered a safe, minimum O2 sat level.
Click to expand...
A point to ponder when considering this statement. The FAA, which was really the CAA at the time, was really taking its information concerning this from the US military. The determination, at the time of these regulations, was based primarily on experimentation on pilots who, almost without exception, all smoked tobacco to the point of multiple packs a day. The O2 saturation levels of a heavy smoker are going to be very drastically different than a non-smoker.
 
RVbySDI said:
A point to ponder when considering this statement. The FAA, which was really the CAA at the time, was really taking its information concerning this from the US military. The determination, at the time of these regulations, was based primarily on experimentation on pilots who, almost without exception, all smoked tobacco to the point of multiple packs a day. The O2 saturation levels of a heavy smoker are going to be very drastically different than a non-smoker.
Click to expand...

But...Those guys were 25. Most pilots are slightly older today. ;-)
 
Bugsy said:
Pressure altitude as stated by others.

Moisture does play a role at altitudes above 28,000ft where the pressures are low enough that water vapor displaces oxygen to a point that we need to deliver oxygen under pressure to maintain a pO2 high enough to oxygen the blood.

But when your thinking about your own personal supplemental oxygen need and when and how you use supplemental O2 think in terms of pressure altitude.

Bugsy, 2015 president aerospace physiology society
Click to expand...

It's true that your lungs have 100% saturated water vapor in them (at 98 F and ambient pressure) that does displace some oxygen. And the amount of that water vapor does increase as pressure drops (altitude increases). But the big effect is still PA decreasing as you climb, until 100% oxygen (minus the displacement by water) cannot maintain sufficient blood oxygen levels. I thought that altitude was around 38,000' (PA)? I need to check.
 
az_gila said:
But may that be driven more by FAA altitude and O2 regulations?
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Oh absolutely. Being old and tired, I turn on my O2 any time at or above 10,000' - even lower on a long flight. My only point was that ForeFlight uses pressure altitude (which comes from my iPad Air 2).
 
Bob, that's the way I understood it as well!

On the purely practical side as opposed to the scientific side, everyone has their own limits with multiple factors unique to each of us. I as a long distance runner can tell the difference between 600 feet where I live and 2100 feet where I just ran a half marathon, or a run acrost the parking lot of Pikes Peek at 14100 ft. I use O2 in my RV at anything above 10000, because I can tell the difference above that for long trips. Headaches, sleepiness, slower than usual thought processing, eye sight, etc. Just me.

Again, what Bob stated is what I was taught.
 
bret said:
Oooo......so it's pressure not density........wait, what?.....if the air is thinner, isn't lighter, and if its lighter there is less pressure???
Click to expand...

The pressure is based not upon the air at your level, it is the air above it. Gravity pulls all of the air molecules toward Earth. The greater the volume of air (up to the point of zero gravity) above your air the more weight is applied to your air and therefore a higher pressure. anything that reduces the weight of the air above you will reduce the pressure of the air at your level, assuming a constant volume (i.e. actual altitude) and vice versa.

Is that clearer or muddier?

Larry
 
BobTurner said:
No. Variations in pressure ALT (vs indicated ALT) are a small effect in calculating density altitude. Non standard temperatures are the biggest effect. Just make up some examples and calculate DA on your e6b to verify.
Edit: Just to clarify, I am talking about how much density altitude varies from indicated altitude. I believe that is common useage, e.g., if someone at KTVL (elevation 6000') says 'DA is high today", they mean in comparison to a standard day at KTVL. In an absolute sense, the DA at KTVL will always be much higher than at a sea level airport.
Click to expand...


And in the absolute sense, the high DAlt on a warm, humid day at KTVL results more from the fact that it sits even on a standard day at 6,000 feet MSL than to all other factors put together. The absolute altitude - and with it the PAlt - is the largest contributor to the DAlt.

I think the takeaway is to: follow the regs, have O2 available on long, high or night flights where it might be very useful even if not a legal requirement, and take advantage of pulse oximetry (while realizing these amazing little gadgets can't correct for anemia, CO poisoning, and a few other rarities).
 
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Had some Aeromed classes when learning how to fly helicopters back in the 1980's.

Some of the things I learned where kind'a interesting:
1) There is always 21% oxygen available no matter how high you go (just reduced pressure)
2) If you live at sea level, a quick climb to 10,000 ft can put you in a hypoxia state
3) Smokers at sea level are constantly at a minimum of 5,000 ASL
4) The smoke from any tobacco products contain carbon monoxide which has an affinity of 300-400 times greater to hemoglobin than oxygen = if CO is available the hemoglobin will pick up CO before O2.
5) If you live at sea level, a climb to 5,000 and your body starts to compensate for the reduced air pressure-heart rate and respirations increase

While at the school we had a day of altitude chamber training. Something that will never be forgotten.

At 25,000 we took our masks off and had the chance to do simple problems, once done, we then turned the paper over and wrote our names until it was time to put the mask back on.

Writing your name seems kind of easy, even looks like some of the best writing you've ever done, however once the mask goes back on, it isn't even good scribble!

I use my oxygen anything above 8,000. Live at 89' ASL.
 
Interesting, a climb to 10K will put you into the effects of hypoxia, what if you live and work at 5 and 7K, how far will the body adapt?
 
Bret,
Good question I would like to know the answer to. I'm 65 and we're considering moving to Pagosa Springs, CO at 7500 ft. Not sure how this old body will cope after living near sea level.
Jerry
 
Adjust

SR2500 said:
Bret,
Good question I would like to know the answer to. I'm 65 and we're considering moving to Pagosa Springs, CO at 7500 ft. Not sure how this old body will cope after living near sea level.
Jerry
Click to expand...

I live at 7,500'. We relocated from sea level. Your body adjusts in a week or two. However, the air is thin and no matter how fit, you still feel it to some degree. I have no problems when hiking at higher elevation and excercise at 9,000' every work day. I drive one of the RMNP shuttles so I see the people who race up from sea level and suffer from altitude sickness because they don't give adequate time to acclimate. Some get really sick. It effects young and old but less to the athletic. So excercise!

We recently came to visit the kids and it's funny. I walk every day and my heart rate and breathing during activity are noticeably slower than home. Now I get to reacclimate again when we go back home.:eek:
 
SR2500 said:
Bret,
Good question I would like to know the answer to. I'm 65 and we're considering moving to Pagosa Springs, CO at 7500 ft. Not sure how this old body will cope after living near sea level.
Jerry
Click to expand...

My neighbor was injured in a snowmobile accident, climbing 11,000 mountains, in the back country Sierras, got crushed by the sled, injured his lungs, had to sell his house and move to a lower elevation to live. Another elderly gentleman I know lives at SL, and must be on O2 when he visits us here at 5K. I would go visit that elevation for a few days and take an O2 reading.
 
Sp02

bret said:
My neighbor was injured in a snowmobile accident, climbing 11,000 mountains, in the back country Sierras, got crushed by the sled, injured his lungs, had to sell his house and move to a lower elevation to live. Another elderly gentleman I know lives at SL, and must be on O2 when he visits us here at 5K. I would go visit that elevation for a few days and take an O2 reading.
Click to expand...

Bret is correct.
I should have mentioned it. Spend a few weeks in Pagosa and monitor your O2 and BP. We keep a meter and BP cuff nearby and check daily. FYI Sweetie and I moved 5 years ago. Both of us were about 55.
 
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