The body sweats and generates heat, especially when there is physical exertion. There are several reasons why these byproducts should be removed. Breathability of clothing refers to how well heat and sweat can be removed. The same piece of clothing may also need to be waterproof against environmental water.
Reasons why sweat and heat need to be removed (and not allowed to accumulate):
(1) overheating in very hot weather
(2) heat loss in very cold weather
(3) comfort, with accumulated sweat causing a clammy feeling
(4) safety if sweat gets onto hands or into the eyes
High-tech layered membranes such as Gore-Tex allows water vapor from the body to escape while keeping liquid water out.
Rating is similar but the opposite of waterproofing, since the goal is to get moisture out rather than keep it from coming in.
Rating using an upright or inverted cup test reports grams of water per square meter of fabric per day.
Rating in grams water per square meter per day
Breathability
Exertion Level
< 5,000
very low
very light
5,000 - 9,999
low
light
10,000 to 14,999
moderate
moderate
15,000 to 20,000
high
heavy
> 20,000
very high
maximal
The Ret scale refers to resistance to evaporative heat loss.
Ret Scale
Breathability
> 30
very low
20 to 30
low
13 to 20
moderate
6 to 13
high
0 to 6
very high
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