[mawea (OP)]

can anyone tell me if there is any negative effect on using oxygen mask (100%)say up to 30mins duration for a normal healthy person ? i have heard of infants becoming blind in incubators.

[bezoar]

In a normal person, without any respiratory diseases, I doubt 30 minutes would be harmful.  Prolonged inspiration of 100% oxygen could lead to pulmonary edema and respiratory failure.

Bezoar

[John Doe]

What about astronauts? Before the Apollo 1 accident the capsule had a mix of 100% oxygen.

[chris]

Despite being absolutely critical for life, oxygen is harmful in high doses because of its ability to produce harmful substances called 'free radicals' . These reactive species damage tissues, including the lung and brain, and their production increases with the proportion of oxygen in inspired air.

Divers are at particular risk because the partial pressure of alveolar oxygen increases with increasing depth.

However, 30 minutes of exposure is unlikely to do you much harm.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[Penguin]

but don't divers breath compressed air, not 100% O2?

[roberth]

Yes, but the oxygen portion of the air compresses at a different rate to the nitrogen (hence the air (oxygen component anyway) becoming toxic below about 150').

Chris, I am under the impression that free radicals are dead red blood cells that haven't been converted and expelled yet. Does excess oxygen intake wipe out red blood cells more rapidly? Don't these free radicals get converted into bilyruben (sp?) as in the white dog poo thread a while ago.

[tweener]

I'm no diver, but I believe that divers that are going to dive very deep or for an extended period of time use a special mix of gases to minimize the toxicity of the O2.  I have a friend that is a diver and she is always complaining that they are limited in how long they can dive in a day.


----
John

[Ylide]

Oxygen has a particular trait in that it's bonds tend to cleave homolytically.  A covalent chemical bond, such as the one in O2, is what happens when two atoms each share an electron with each other to fill an orbital.  (as opposed to ionic bonds like in salts where the bonding electrons all come from one atom.)

Bonds are usually cleaved heterolytically, so that one atom keeps both electrons from the bond. (or one of the electrons goes away entirely like in redox and electrochem reactions)  But, oxygen has a tendancy for homolytic bond cleavage where each oxygen atom keeps one electron.  This is called a free radical.  Any atom or molecule with 1 and only 1 unbonded electron is a radical.  They are VERY reactive and do bad things to cellular structures, especially DNA.  They can also initiate nasty chemical reactions like breaking water molecules apart to form perm hydrogen peroxide...a nasty cell toxin.  

Oddly enough, this trait of oxygen that makes it cleave this way (quantum mechanical explanation) also causes it to be affected by magnetic fields in liquid form.  One of the coolest things (no pun) I've ever seen is a ball of liquid oxygen suspended in midair, spinning and throwing off vapor.

It's a shame we need oxygen to end the electron transport chain, because it's not a particularly nice substance when it comes to our physiology.

As far as it's direct acute toxicity, I'm not quite sure how that occurs.  I've heard of divers using a technique called saturation diving, but I really don't know exactly what that means other than they have to stay at a certain depth for a certain period of time to saturate their blood with gases so that it doesn't oversaturate at greater depths.  



This message brought to you by The Council of People Who Are Sick of Seeing More People

[chris]

The role on the helium / oxygen mix used by deep sea divers is partly to overcome the high resistance to airflow under conditions of high pressure.

for every 10 metres you descend under water the ambient pressure increases by 1 atmosphere; accordingly your dive regulator delivers air at 2 atmospheres pressure to compensate with the result that you are now breathing air that is twice as dense as air at the surface. Descend to metres and the air you breathe is 3 times as dense (contains 3 times as many air molecules) as at the surface. Because the water pressure is much higher at depth the volume of the air remains the same. But if you take a deep breath and then ascend rapidly the air expands as the water pressure drops and your lungs try to inflate to 3 times their normal size and explode.

But that's an aside. The crucial point is that as you keep going deeper, and the air you are breathing becomes denser and denser are the water pressure rises, eventually you reach a point where the air you are breathing is so 'thick' that it almost has the consistency of a liquid and feels a huge resistance from the airways as it tries to enter and leave the lungs. The work of breathing becomes too great to sustain. It's a bit like having severe asthma. Helium is much lighter and less dense. By replacing the nitrogen in air with helium you make it easier to breathe by reducing the airway resistance.

Occasionally anaesthetists use this trick to bypass airway obstructions (like an inhaled foreign body) whilst they try to remove the cause of the problem. The helium mix can slip past the obstruction more easily...

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx