Sunday, July 13, 2008

Apple #328: It's the Humidity

The other day, I was temporarily without a car, so I was taking the bus to work. I had already ridden on one bus and was waiting at a stop for the second. It was a sunny day and warm, but not especially hot, and every once in a while a nice little breeze came up with a lovely cooling effect. But though the temperature wasn't hot, the air was very humid. I was standing still, reading some poorly written free newspaper, and the sweat was rolling off of me as if I'd been running for miles. I was very glad when the air-conditioned bus arrived and I could get out of that humidity.

(Image from the City of Ann Arbor)

I've noticed that high levels of humidity bother me now more than they used to when I was younger. So I'm curious about a lot of things having to do with humidity: why do we perspire more when it's more humid? Do high levels of humidity somehow put more of a strain on our bodies so that we feel more tired as a result? What are the effects on our health when we're in very humid environments?

  • Actually, it is incorrect to say that we sweat more when it is more humid. Our bodies produce the same amount of perspiration whether it is humid or dry; what makes us sweat more is higher temperatures.
  • To understand why we feel like we sweat more when it's humid, it's useful to review how sweat works in the first place:
  • Sweat is our body's way of cooling us off when it gets hot.
      • Sweat glands release a mixture of water and various salts and potassium through tiny ducts up to the surface of the skin.

Sweat gland and its duct leading to the skin's surface
(Diagram from Skin Care Forum)

      • Once that water is up there on the skin, the water discovers it's pretty warm out there, maybe even warmer than it was in the body, and it wants to evaporate.
      • It is a pretty big deal for any element of matter to change states -- in this case, for liquid (sweat) to become a gas (water vapor) -- and it therefore requires a decent amount of energy to make that conversion happen. In this case, the heat of your body provides that energy, and the sweat therefore evaporates.

Simple diagram of how evaporation happens as the temperature goes up
(Mark Swindle, World Book, sourced from NASA)

      • The clever part is that in that evaporation process, the heat from your body has been "spent," so to speak, on the evaporation. So not only has the sweat evaporated, but your body has also been cooled off by the amount of energy it took for the evaporation to occur.
  • This process doesn't work quite so neatly when the air is very humid. When it's humid, there is already a lot of moisture in the air, and unless the temperature goes up so more, no more water will evaporate into it. So the sweat on your skin stays there. Because it doesn't evaporate, your skin doesn't get to spend its heat-energy on the evaporation process. So now instead of being cooled off by sweating, all you feel is hot and damp.

So what is too much humidity? First, allow me to clarify what most people mean when they say "humidity."
  • If you look at a weather report than includes some mention of humidity, it will usually display it in terms of a percent. The figure they're giving you is actually the relative humidity.
      • Relative humidity is the amount of water vapor present in the air divided by the amount of water vapor that could possibly be present in the air.

This diagram demonstrates the concept of relative humidity, how higher temperatures can accommodate more water vapor.
(Diagram from Physical

      • The higher the temperature, the greater the amount of water that could be evaporated into the air (turned to vapor).
      • So in order to figure relative humidity, you have to know three things: 1) the amount of water vapor in the air 2) the current temperature 3) the amount of water vapor that could possibly be in the air at that temperature.
      • Let's say you've measured the amount of water vapor in the air and discovered that there are 20 grams of water per cubic meter of air. The temperature also happens to be 86 degrees F (30 C). Meteorologists tell us that air at that temperature "can hold" 30 grams of water per cubic meter of air. So you'd calculate the relative humidity by dividing the amount of water vapor present (20) by the amount possible (30), and you'd get 66.6%.

So what does that mean? So what if the relative humidity is 67%? Who cares except a devoted meteorologist and the Apple Lady and maybe one or two other nuts?
  • The reason you do care, probably without realizing it, is that most people are comfortable at relative humidity levels around 45%. If the relative humidity is at 67%, the sweat that your body produces won't evaporate very well and you will be experiencing more discomfort from the heat as a result.
  • Furthermore, lots of things that make people allergic are more active at humidity levels above 50%. Molds are thriving and releasing their spores in that moisture-rich environment, dust mites are happily rolling around their balls of dust and fecal matter, and all sorts of pollen are hanging around in the air on those particles of water vapor. So if you've got allergies, you're probably going to discover you're sneezing and coughing more often if the humidity is over 50%.

You might find yourself doing this more often when it's more humid
(Photo from EHS Innovators)

  • Furthermore, people's asthma tends to get triggered more often when relative humidity is over 50%. Bacteria and viruses love to thrive when the humidity ranges from 50% to 70%. If you've got arthritis, you're going to feel the ache more at humidity over 50%.
  • The higher the relative humidity, the more susceptible you are to things like heat stroke, heat rashes, or heat exhaustion. This is because, as we've discussed, at higher humidity, your body's cooling system doesn't work as well, so the heat has a greater effect on your body at lower temperatures.
      • Meteorologists have a method of expressing this, called the Heat Index. It's based on quite a complex calculation, but it is a function of the temperature and the humidity, to provide an expression of how hot it feels.
      • In our example, the temperature is 86 degrees F and the relative humidity is 67%. According to NOAA's Heath Index calculator, that means those conditions will feel like 94 degrees F. And at 94 degrees, your body heats up and gets tired a whole lot faster than it does at 86 degrees.

Here's another way of depicting the heat index. The humidity and the temperature work together to raise the heat index.
(Chart from NOAA's National Weather Service)

If you can get the relative humidity below 50%, you'd be so much more comfortable for this army of reasons (dust mites tend to die off, molds get less active and dry up, perspiration is more likely to evaporate, etc.). So how do you reduce the humidity?

You could move to a part of the planet where there is less humidity. On this map, dark gray areas tend to be humid. Yellow and tan regions are arid (dry).
(Map of the Global Humidity Index from the University of Arizona Office of Arid Lands Studies)

  • Well, you can't do anything about the air outside. But you can do something about the air inside.
      • You can run an air conditioner, which runs hot air over condenser coils and removes both heat and moisture (this is why you get water dripping from an air conditioning unit in your house or in your car).
      • Or you can run a dehumidifier, set at around 45%. A dehumidifier works very much like an air conditioner, except it is even better at reducing the moisture in the air. Sometimes even a central A/C unit will work more efficiently with the aid of a dehumidifier.

The LG 45 Pint Dehumidifier with Electronic Controls got one of the highest customer ratings from Epinions, and you can get it through Amazon for about $110.

  • When you are outside or are otherwise in high-humidity conditions, you can take steps to keep yourself from getting overheated:
      • Stay hydrated to replenish the moisture you're losing to perspiration.
      • Wear loose-fitting, lightweight clothes, preferably made of cotton. You don't want to be wearing fabrics that will trap and keep the heat and perspiration close to you.
      • If you're exercising or doing strenuous work, give yourself time to adjust. Start slowly and increase your pace slowly, and also allow for a longer than usual period to cool down.
      • Allow yourself to take breaks.
      • Wear a hat and sunscreen, even if it's cloudy.
      • If it's at all possible, avoid strenuous work or exercise in the hottest part of the day. (Most people think this is around noon, but actually, this is around 5 to 6 pm.)

Spreading asphalt has always looked to me like one of the hottest jobs ever.
by Matt Smith from LeHigh Valley Live)

MadSci Network, Why do people sweat more in humid weather?
Howstuffworks, Why We Sweat and What is relative humidity and how does it affect how I feel outside?
Jack Williams, Getting a handle on humidity, USA Today, July 18, 2005
Duane Johnson, "Hate the Humidity?" housekeeping channel
Relative Humidity and Health, Engineering Toolbox
US Dept of Energy, A Consumer's Guide to Energy Efficiency, Your Home, Central Air Conditioners
EmaxHealth, Exercising Safely in the Summer Heat and Humidity


  1. This post was really usefule. Thanks!

  2. You're welcome. Thanks for letting me know!

  3. Dominik, why did you find this entry not useful? Did you have a question that didn't get answered? Without more information, your comment is not useful, either.

  4. What is the topic for this?

  5. You can actually find the Heat Index by the dew point temperature.

    If the dew point is:(F) 50 55 60 65 70 75 80 85
    You add this to the: 2 5 8 11 15 19 25 31


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