Could Deforistation lead to lack of ability to light fires?

[wasteofo2]

The rainforest produces a good amount of o2 (exactly how much I'm not sure), could deforistation of the rainforest lead to such a decrease of o2 in the air that fires wouldn't be able to light? About how much could the percent of o2 in the air vary before fires stopped being able to burn?

 

[Chemicalsuperfreak]

Originally posted by wasteofo2
The rainforest produces a good amount of o2 (exactly how much I'm not sure), could deforistation of the rainforest lead to such a decrease of o2 in the air that fires wouldn't be able to light? About how much could the percent of o2 in the air vary before fires stopped being able to burn?

Most oxygen in the air is produced by algae, not rainforests. If all of the oxygen production in the world was stopped, and the remaining oxygen was completely consumed, then sure forest fires wouldn't be able to light. But then again we'd all be dead. Trees included.

 

[ShawnD]

Don't trees consume as much oxygen at night as they produce during the day? I heard something like that somewhere...might have been TV.

 

[wasteofo2]

Yes, algae produces about 70% of the worlds o2 and the rainforests produce about 20%. I'm not saying eliminating all the o2, but if let's say, the current amount of o2 in the air (about 21% or so, right?) was reduced to something like 19%, humans would, as a whole, be fine.

Bassically what I'm asking is: what percent of the air does o2 need to make up for fires to burn?

 

[ShawnD]

Depends entirely on air flow. If you have massive amounts of air flow, you can have a roaring fire with very low concentrations of O2. If you have poor air flow, higher concentration of O2 in the air is needed.

 

[wasteofo2]

what about in your house, with presumably no airflow or very little, perhaps that caused by central air, a fan or a draft.

 

[chroot]

We need about 0.16 atmospheres partial pressure of oxygen to remain conscious.

- Warren

 

[wasteofo2]

Care to put that in terms of percent, or does that bassically mean 16%?

 

[chroot]

Originally posted by wasteofo2
Care to put that in terms of percent, or does that bassically mean 16%?

You can't just express it in percent -- it depends on the pressure. For example, at 1 atmosphere of total pressure, here on the surface of earth, I need a 16% fraction of 02 to stay conscious.

If I dive to 33 feet in seawater, I am exposed to 2 atmospheres of pressure. There, I can survive with only an 8% fraction of 02.

- Warren

 

[ShawnD]

16% assuming that the air pressure is at 1 atm.

 

[wasteofo2]

Alright, I understand that, so bassically if all the rainforests were completely destroyed without any other loss of vegetation we could just remain concious

 

[chroot]

Or we could pressurize our living environments.

- Warren

 

[wasteofo2]

Originally posted by chroot
Or we could pressurize our living environments.

- Warren

By doing what, living in underwater colonies, building domes around cities?

Do you happen to know the atmospheres partial pressure needed to light a fire?

 

[chroot]

Originally posted by wasteofo2
Do you happen to know the atmospheres partial pressure needed to light a fire?

Depends on the fuel.

- Warren

 

[wasteofo2]

the fuel being oxygen in the air, or did you mean like wood/oil?

 

[chroot]

The oxygen is (not suprisingly) the oxidizer. The fuel is the wood, oil, or other substance which is oxidized by the oxidizer.

- Warren

 

[wasteofo2]

Well how much difference is there between different sources of feul, for instance, wood and oil.

 

[chroot]

A huge difference.

- Warren

 

[wasteofo2]

So how much atmospheres partial pressure do you need to sustain a wood fire and an oil fire?

 

[ShawnD]

Why do you thik he would know that? It's not exactly common knowledge.

 

[russ_watters]

I can't answer the question directly, but try THIS site for the opposite approach. At an altitude of about 5 miles, you're above 50% of the atmosphere so the pressure is half of what it is sea level. AFAIK, climbers in the Himalayas (sp) don't have problems lighting fires.

 

[wasteofo2]

Originally posted by ShawnD
Why do you thik he would know that? It's not exactly common knowledge.

Practically nothing discussed on this board is common knowledge. I just kinda figured how much o2 pressure you'd need to light a fire on wood would be relatively common knoweldge among people who talk about the ridiculously advanced things which are discussed on this board.

 

[Chemicalsuperfreak]

Originally posted by wasteofo2
Practically nothing discussed on this board is common knowledge. I just kinda figured how much o2 pressure you'd need to light a fire on wood would be relatively common knoweldge among people who talk about the ridiculously advanced things which are discussed on this board.

Actually most of what is discussed here is common knowledge. It's mostly at the level of undergraduate science classes. Suplemented here and there with more expert opiniongs.

How much O2 you need to light a fire is not common knowledge. Furthermore, your original question pertained to forest fires. Most of which, I believe, are triggered by lighting strikes. And I think a bolt of lighting could probably start a fire with very little oxygen. So as far as forest fires are concerned, I think the question isn't so much about starting the fire as it is sustaining it.

It's further complicated by the fact that as oxygen is reduced, the fire doesn't just go out, it is slowed.

Frankly, I think the lower limit of oxygen needed for wood to burn is quite less then that needed for aerobic organisms to breathe. Cut the oxygen level too low and you'll have a bunch of dead trees burning slowly.

 

[wasteofo2]

Originally posted by Chemicalsuperfreak
Actually most of what is discussed here is common knowledge. It's mostly at the level of undergraduate science classes. Suplemented here and there with more expert opiniongs.

Mmm, I realize that, it just seems that if you understand all this stuff which is so far beyond what the average joe can even begin to imagine, that how much o2 you need to light a fire would be fairly common knoweldge, I guess I was wrong.

Originally posted by Chemicalsuperfreak

How much O2 you need to light a fire is not common knowledge. Furthermore, your original question pertained to forest fires. Most of which, I believe, are triggered by lighting strikes. And I think a bolt of lighting could probably start a fire with very little oxygen. So as far as forest fires are concerned, I think the question isn't so much about starting the fire as it is sustaining it.

I didn't say anything about forest fires, i referenced the rainforest as an o2 source, sorry if that somehow got misconstrued.

Originally posted by Chemicalsuperfreak


It's further complicated by the fact that as oxygen is reduced, the fire doesn't just go out, it is slowed.

Frankly, I think the lower limit of oxygen needed for wood to burn is quite less then that needed for aerobic organisms to breathe. Cut the oxygen level too low and you'll have a bunch of dead trees burning slowly.

So bassically we'd all be dead before lighting fires became a problem?

 

[wasteofo2]

Originally posted by russ_watters
I can't answer the question directly, but try THIS site for the opposite approach. At an altitude of about 5 miles, you're above 50% of the atmosphere so the pressure is half of what it is sea level. AFAIK, climbers in the Himalayas (sp) don't have problems lighting fires.

That seems to contradict what chroot said, unless I misunderstood one or both of you, or my assumptions about mountain climbing are wrong.

As understood:
It's not disputed that tehre's approx 21% o2 in the air.

Chroot said you need 16 atmosphere's partial pressure of o2 to stay concious.

You're saying that there's 50% atmospheres partial pressure 5 miles up, which means that there's really only about 10.5% atmospheres partial pressure of o2.

I know mountain climbers take o2 tanks with them, but i didn't think they are constantly using them, and I'm pretty sure I recently saw photos of everest climbers who were supposed to be about 16,000+ feet up with no o2 masks on doing work.


So, where am I, you or chroot wrong?

 

[russ_watters]

Originally posted by wasteofo2
So, where am I, you or chroot wrong?

Mountain climbers are in much better than average shape.

 

[chroot]

Very few people, if any, can survive at 25,000+ feet with no external oxygen supply. Some people may be able to survive on as little as 0.10 atmospheres partial pressure of O2, but probably not for long.

- Warren

 

[wasteofo2]

Originally posted by chroot
Very few people, if any, can survive at 25,000+ feet with no external oxygen supply. Some people may be able to survive on as little as 0.10 atmospheres partial pressure of O2, but probably not for long.

- Warren

There was just a thing on the history channel about some early attempt to reach the sumit of everest and they said that over 16,000 feet the body starts to deteriorate. Not sure, however, whether it's due to lack of oxygen or the low temperature or the lack of air pressure or a combination of all of them plus other variables.

 

[chroot]

Well, the lack oxygen is a HUGE part of it, certainly.

BTW, my figure of 0.16 ATA PPO2 (atmospheres absolute partial pressure O2) is what is usually used by advanced mixed-gas technical scuba divers (like me). We don't mix or use any breathing gas that will produce < 0.16 ATA PPO2 at its working depth. This means that 0.16 ATA PPO2 is enough to support virtually anyone for virtually any length of time. There are certainly people who can operate just fine at 0.14, or 0.12, or 0.10 -- 0.16 is just a conservative figure, with some "safety margin" built in.

And to answer your earlier questions, there are a couple of relevant points:

1) Define "fire." Do you mean "the chemical reaction in which fuel is oxidized?" Or do you mean "flame?" The chemical reaction can exist at almost ANY partial pressure -- it'll just run very very slowly, as has been pointed out.

2) The actual partial pressure necessary for a fire to grow given some fuel is strongly dependent upon many, many factors. You can't assign a particular PPO2 to wood, say, because big logs, little sticks of kindling, and sawdust all present greatly different surface areas to the atmosphere. An oil that has a high vapor pressure will burn more easily than another. An oil that has been atomized (sprayed into tiny droplets) will burn easier than an oil left in a vat of liquid, and so on.

Because fire is so sensitive to the geometry, composition, etc. of the fuel, it is not possible to assign a particular PPO2. The devil is in the details.

- Warren

 

[wasteofo2]

When referencing fire, i was thinking more like the types of small fires or sparks you need to do things like fire a gun or start an engine, but I've pretty much gathered that we'd all be dead before cars didn't start due to lack of oxygen.