[sue howarth]

sue howarth asked the Naked Scientists:
   
Dear Dr. Chris and colleagues,

Several days this week I have noticed the humidity level has been 80% plus and almost every day it is more than 60%. As far as I know, this means the air we breath contains 80% or 60% water. I also know that with too much water in the lungs we drown.

Why then, do we not drown, when we are constantly breathing in so much moisture?

I listen to the programme on BBC Radio Suffolk, every week-end at 6pm and would really appreciate an answer to the above.

Yours sincerely,

Sue Howarth

What do you think?

[LeeE]

We don't drown because the humidity figures you're referring to are for Relative humidity and not Absolute humidity.

Absolute humidity is measured by mass/volume, usually grams/cubic metre, but Relative humidity is measured as a percentage and is where the 80% & 60% numbers come in.

Relative humidity is defined as the ratio of the partial pressure of water vapour (in a gaseous mixture of air and water vapour) to the saturated vapour pressure of water at a given temperature.

For water to precipitate out of air and form droplets, in the form of dew or fog/cloud, it is not just the water content that matters; the pressure also plays a major part.  If you look at a cheap disposable gas lighter you can see that the gas inside is in the form of a fluid but if you press the release/trigger down, without lighting the lighter, only gas comes out.  The gas inside the lighter is in the form of a fluid because it's under pressure, but once it leaves the lighter and is at room pressure it turns back into a gas.

The pressure of the air then, is important with regard to the point at which the water vapour in a volume of air will start to precipitate, and that's why relative humidity figures are given - they tell you when something is likely to happen, whereas the absolute humidity won't as it omits the pressure factor.

[lyner]

Whatever the humidity, there are plenty of Oxygen molecules available in contact with the surface of the lungs so the gas exchange would not be affected too much. The waste CO2 will just as easily pass out.
To drown, you would need to have a liquid water or foam, rather than separate water molecules or droplets in your lungs. I think you could react very violently if the air had too many droplets in it - the irritation could cause you to cough or (I guess) have an asthmatic-type of reaction.

[Karsten]

 I read somewhere ("The Perfect Storm" I think) that in certain nasty weather conditions the water surface is so whipped up by extreme winds that swimmers are not able to breathe enough air although their heads are over the water surface. They essentially drown while swimming even if they wear a life jacket and never go under. Never been in this situation (obviously) and hope to never be there, but this thread reminded me of that part of that book.

[lyner]

That sounds very feasible. Horrible way to go!

[LeeE]

It sounds plausible, not because they couldn't inhale enough oxygen, but because they'd also inhale too much water and probably cough themselves to death.

[Andrew K Fletcher]

Some info that might be useful.

People drown on ventilators in intensive care units frequently because the humidity of the air is set far too high!

Respiratory infections generally follow poor removal of fluids from the lungs. When the body temperature drops by 2 degrees during the night around 4.30am sleeping flat, clearing fluid from the lungs becomes more difficult. This is around the time that sudden deaths take place, particularly in the elderly population. Obviously this time will alter when people go to bed earlier or later.

People with multiple sclerosis have relapses while showering or taking a hot bath

A team of elite soldiers abseiled down a waterfall into a high humidity valley in Borneo and found that all of their energy was sapped from them within minutes. They struggled for days to drag themselves out of the ravine and thought they were never going to make it out of there alive.

A questionnaire sent to people with multiple sclerosis revealed that high humidity was the worst environment for them to be in.

Tropical diseases, particularly in river valley high humidity areas are more common than diseases in moderate humidity areas. Two studies in the Rhone valley and another valley revealed that these areas had far higher levels of M.S than the national average.

Leslie Munro did a statistical analysis of sudden infant deaths in river valley and low lying coastal areas. What he found was that the prevalence of cot deaths was way over the National average. Munro attributed this to water winter-logged soils. The Foundation for Sudden Infant Deaths ignored his research.

[Nizzle]

So high humid areas are more favorable to micro organisms. They cause diseases, not the humidity itself.

With statistics you can prove and disprove everything, depending on how you interpret the numbers.

[tuttut]

A team of elite soldiers abseiled down a waterfall into a high humidity valley in Borneo and found that all of their energy was sapped from them within minutes. They struggled for days to drag themselves out of the ravine and thought they were never going to make it out of there alive.

There is a difference when the temperature is great and so is high humidity, and when the temperature is low at high humidity.

A questionnaire sent to people with multiple sclerosis revealed that high humidity was the worst environment for them to be in.

So buying a dehumifyer and sitting indoors would be seen as a cure?

Can you confirm that the questionnaire asked people where they got their humidity readings from? If they were from a website for a given geographic area then they are not using a true reading. That stated on a webpage even those operated by The Met Office and the NWS will be out by a few percentage points depending on your location to the nearest observation station.

Did they all note the readings at the same time of day? The time of day the reading is taken will give different RH readings.

Did they take their own readings? If they did were they in or out of doors? Again, this will give different reading.

If these, and more were not controlled then their outcome really means nothing.

Can you explain why, when the Relative Humidity is 100% and the dew point has been reached, that I do not drown in Fog?

Everything you state is related to relative humidity, Relative humidity really means nothing because it's relative! You really need to read what LeeE has written. You should also know that barometric pressure is also a factor and that the specific humidity is is a measure of the barometric pressure and the mole fraction of the water vapour in the air.

[Andrew K Fletcher]

The humidity questionnaire merely asked people with ms what weather causes their condition to worsen. High humidity came out on top. You are welcome to split hairs about how this was conducted it is your own prerogative to do so.

You may even disregard it.

Andrew

[Andrew K Fletcher]

So high humid areas are more favorable to micro organisms. They cause diseases, not the humidity itself.

With statistics you can prove and disprove everything, depending on how you interpret the numbers.

Maybe the high humidity also lowers our body's resistance to pathogens?

[LeeE]

None of this really relates to drowning directly due to the water content of the atmosphere though, does it?

[tuttut]

None of this really relates to drowning directly due to the water content of the atmosphere though, does it?

No. But some things do need clearing up. Would you agree?

...You are welcome to split hairs about how this was conducted it is your own prerogative to do so.

Andrew

it's not splitting hairs. If they were basing it on false data or more likely data they did not understand then is there Any worth in what they think?

Say they checked their local forecast in the AM and took that reading of humidity and based their medical state on it, then it is worthless because the RH changes throughout the day. It is likely that many people lived 30+ miles from their nearest observation station. It could be foggy there with a RH of 100% and clear at their home with a RH of 70/80 they are not using their own values at their location.

I'm not being an arse, bur most of the general public don't know the diffrence between the different measures of humidity, most don't know what the RH value represents, let alone dew point. So the results of a questionnaire has little meaning when the participents don't understand what the meteorolical values mean or relate to. Do you agree?

Edited for the obvious and numerous spelling mistakes.

[Andrew K Fletcher]

Just remembered. It was a long time ago when this was conducted. But there was a pilot study involving a small group of people with ms who were given a humidty meter, only a cheap and chearful one that is used in greenhouses and cost about £1.50 but from memory they were reporting humidity readings over 85 during relapses and some people did not realise that drying washing on the radiators was causing the problem.

Maybe a controlled study could be conducted by someone to give more useful data. Any PHD students want to follow this up?

[Bored chemist]

In this thread
http://www.thenakedscientists.com/forum/index.php?topic=16414.msg260067
where Andrew went on about the "interesting" idea that wet weather caused Foot and mouth disease in farm animals, I pointed out that air leaving the lungs is (practically) always wetter than the air going in.

The office where I work has  low humidity and causes real problems for some people.

It would be silly to sugest that weather doesn't affect people's health but we ought to stick to real science here. Since relative humidity is strongy dependent on temperature it's not, on its own, a valid measurement of the amount of water in the air. If some illness is found to occur more often at high RH it might mean nothing more than that it is favoured by cold weather.

[LeeE]

Relative Humidity (RH) is strongly dependent upon temperature and pressure: you shouldn't forget the pressure factor.

BC: I think there's a small blooper in your last sentence:

If some illness is found to occur more often at high RH it might mean nothing more than that it is favoured by cold weather.

Surely, if an illness if found to occur more often at high RH then would it not seem to favour hot/warm weather, rather than cold weather?

Anyway, while the RH will affect the rate of transfer of water between the air and other absorbent materials, so that your laundry will dry more quickly in low RH and a piece of dry blotting paper will absorb water more quickly when the RH is high, you also need to consider the effects of pressure when you start thinking about how the human body is affected.

In terms of health issues, I think we also need to be careful of exactly what health issues we're talking about here i.e. are we talking about the probability of 'catching' something or are we talking about the effects of RH and pressure on existing conditions?

Afaik, there's little correlation between catching colds and being cold and what I think we're really talking about here is the effects of RH and pressure upon existing conditions i.e. Multiple Sclerosis or Arthritis.

I don't personally know about the effects of RH, temperature and pressure upon MS but if people with MS find that taking a hot shower or bath can cause a relapse then I'm happy to accept that.  What I can say though, is that my Arthritis does seem to be affected by the weather, and moreover, by the pressure rather than the temperature or RH.  In fact, my Arthritis seems to be worse not when the temperature, pressure or RH is high or low but when it's changing; once the weather settles down to a steady condition again, regardless of whether it's hot or cold, wet or dry, the aches from my Arthritis eases off.

[Bored chemist]

"Surely, if an illness if found to occur more often at high RH then would it not seem to favour hot/warm weather, rather than cold weather?"
Nope.
If I take some air and heat it up the relative humidity falls, conversely, if I cool it the relative humidity rises.
The highest relative humidity you get is a fog at 100% RH and that's generally during cold weather.
Also air at 20C and 100%RH contains (IIRC) about 17mg of water per litre and it does so whatever the atmospheric pressure.
Changes in the pressure can change the relative humidity- it's not a simple relation.

[LeeE]

But do you not tend to get higher RH in warmer/hotter weather than in cold weather?

When fogs form as a result of a drop in temperature, that air cannot subsequently take up the same amount of water again without being heated: once temperatures have dropped, and then stay low, you'll end up with lower RH until the air warms up.

I can't see either, how you can say that air at 20C and 100% RH contains the same quantity of water regardless of pressure.  Pressure is an intrinsic factor in RH, so if two volumes of air are at the same temperature and have the same RH they must be at the same pressure.

[Bored chemist]

Did you understand that you tend to get fog when it's cold?
That's the highest RH you can get.
If I take some air and cool it the RH rises so it's easier to get high RH with cold air.
As for
"Pressure is an intrinsic factor in RH,"
no it's not.
This
http://en.wikipedia.org/wiki/Dalton's_law
tells you that the amount of water you can get in the vapour phase is the same whether there's any air there or not.

[LeeE]

Are you just being sarcastic BC?

Did you understand that you tend to get fog when it's cold?

How did you manage to not interpret: "When fogs form as a result of a drop in temperature,..", and specifically "drop in temperature" as not meaning cold, or at least relatively so?

...so yes, I did understand that.  Did you simply fail to read or think about what I wrote?

I'm not disputing the fact that if you cool air you will increase its RH but what we were talking about were the weather conditions where you'd be likely to get high RH.  So yes, if there is a sufficient drop in temperature the RH will increase and fogs may form, but this is a transient effect that only happens when the temperature is changing.  Once it's winter, and the weather is consistently cold you will get relatively few fogs because once the air has cooled and the moisture it was carrying has precipitated out it won't be able to pick up enough water for more fog to be created unless the air is heated up and then cooled again (or the temperatures drop below their normal minimum).

In real life, where in the world do you think that the relative humidity is most often low and where do you think that relative humidity is most often high - in the arctic/antarctic regions, or in tropical rain forests?  Which of these environments tends to be warm and which tends to be cold?

Dalton's law, as stated in that wiki article says that:

the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture

which is fair enough, but what do you mean by

whether there's any air there or not

How can we talk about the weather if you think that the presence of air isn't a factor?  Is it not air that we were are talking about?

Incidentally, this wikipedia article http://en.wikipedia.org/wiki/Relative_humidity says that RH is defined by...

the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a prescribed temperature

Perhaps you missed that quote the first time I used it in this thread but I think that you must agree that by that definition, pressure is an intrinsic factor.

Are we actually talking about the same thing, or are you just looking for an argument?