Relative humidity is misunderstood by even those of us who live with it daily in our chambers and in our home lives. Some days there’s dew on the grass, other days there’s none, or there’s a sudden humidity spike in our chamber for no apparent reason. Dew on the grass has very little to do with my chambers, but the dew gets my shoes wet, that’s annoying, and high humidity in my chamber causes my samples to fail, that’s annoying. Ahh, there is a connection. The same ferocity with which the water attacks my feet is attacking the sanctity of my seemingly airtight sample containers—if only water molecules were bigger. Ferocious little buggers!

When I asked my wife how much gas was in the car, she said it was down about a quarter tank since she filled it last, so I easily knew that I had about 75% of a tank relative to a full take. When I hear that an auditorium is at 40% capacity I envision a lot of empty seats, and I’m comfortable with what 40% means relative to a full venue. Anyone can have the same level of comfort when thinking of water molecules zipping around banging against the walls of HDPE sample containers.

How does capacity relate to humidity? At any given temperature Earth’s atmosphere can hold a certain concentration of water vapor where any attempt to make it more concentrated at that temperature will only cause the vapor to condense out of the air as liquid. That maximum concentration is what we’re calling “100% full” or 100% RH. So, if you have a room at 40C/75%RH you have 75% of that room full of water vapor. At the same time, if you have a 30C/75%RH room it is also “75% full”. This is the same principle as slowly adding table salt to water until at some point it won’t dissolve anymore.

You may have done a science experiment at home or in school where, after adding a ton of table salt to water, you then cooled the concentrated salt solution until crystals formed. The warmer water could hold the higher concentration, but in cooler water it had to drop out of solution. Relative humidity is a very similar principle—warm air holds more water vapor than cold air, so the concentration of water vapor in the air at 100%RH at one temperature is different than at 100%RH at another temperature. Any percentages of that are also less. For example, the concentration of water vapor in air at 40C/75%RH is actually much less than the concentration of water vapor in air at 30C/75%RH – in fact it’s nearly twice the concentration!

Let’s look at real concentration amounts for water “dissolved” in the air. The following table shows g/Kg of water/air at various temperatures and percents RH:

Absolute-Humidity-air-CF.png (967×836) (

Based on the above table, one Kg of air at 40C can hold a maximum of 48g of water but at 75% of full you only have 36g “dissolved” (for those non-math wizards, 36 is 75% of 48). So, at 40C/75%RH all of the moisture is in “solution”—it’s safely “dissolved” in the atmosphere of the chamber, and there’s room for 12 more grams of water vapor per Kg of air before it is saturated and starts to “crystalize” out in very watery droplety “crystals.”

When you’re working or just standing in a 40C/75%RH room the moisture in the air is oppressive, it condenses on your glasses because they’re cold, you’re sweat doesn’t evaporate from your arms because your skin is somewhere between 30 and 37C while the chamber air is at 40, so water is actually condensing on your skin and clothing and will continue to do so until you’ve lingered inside long enough for them to get as warm as the room. At this point, unless you are climatized to Brazil or Tanzania, I’d suggest you get out for your safety.

OK, you’re out of there. You’ve pulled your sample from the accelerated chamber and now you’re going to pull a sample from the 25C/60%RH room. That room feels like a nice fall day because it’s cooler and a mere 1/3 of the water molecules are zipping around your skin in the Zone II long term climate than in the accelerated climate. It’s always been a very interesting fact to me that the air in a 40C/75%RH chamber has three times more water vapor than a 25C/60%RH chamber—about 36g vs. 12g of moisture per Kg of air. Those values are based on the table above. So, in an accelerated stability chamber, not only is the temperature stressing your samples, but water molecules are also bombarding every possible little nano-pore that might lead into the interior of your very dry sample. In addition, osmotic pressure is added to the push to get the two water vapor concentrations (inside and outside of your sample) equalized. At 40C/75%RH your sample is going to fail at some point, the meteorological gods will make sure of it. But wait, let’s trick them and stop our study at 6 months. Yay!

So, to summarize, a 25C/60%RH room holds 60% of the moisture that it could hold before water would start dripping down the walls—that is, 12g of the 20g/Kg of air it could hold. The atmosphere in a 30C/75%RH chamber contains 20g out of the 27g/KG that it could hold. OK, enough of these warm temperatures, what about the refrigerator? Well, at 5C the atmosphere can hold about 5g moisture per Kg of air and at -20C it’s a mere 0.8g/Kg. No wonder the walls of your freezer are frosty – every time the door opens, and moisture filled air from your comfortable laboratory is let in, that air then cools, condenses, and freezes on the side of the chamber.

An important question that I was dealing with for some time at a previous position was how important the relative humidity was vs. the absolute humidity (the actual concentration of water in air). I mistakenly assumed that any moisture in my refrigerator would have negligeable effect on my samples since the actual water concentration was so low. I had 80%RH in my refrigerator (Yes, we actually measured it with portable devices, but don’t ask why it was so high.). We needed to know how that compared to another 5C unit that only had 15%RH? I figured since it was only a matter of 4g/Kg vs. 1g/Kg the overall impact would be negligible or even immeasurable given my tightly stoppered glass containers, and especially compared to something like a room temperature or accelerated chamber. But to my surprise and disappointment we experimentally learned that the actual moisture uptake of hygroscopic material (in tightly stoppered glass containers) in the high humidity refrigerator was essentially equivalent to the moisture uptake in the 25C/60%RH chamber. Shocking! A full third the actual water content in the air but the same availability of moisture on my samples.

So, samples exposed to 80%RH at 5C have as much access to moisture uptake as samples exposed to 60%RH at 25C even though the refrigerator had 4g/Kg water in the air and the room temperature unit had 12g/Kg water in the air. Bottom line: the relative humidity level matters much more than the absolute humidity concentration.

Here’s another misconception I had: We would experience occasional relative humidity spikes in our sample handling area due to unplanned steam shutdowns. Our room was set at 22.5C and 40%RH, but when the steam shut down, the room would drop to 15C for at least a few hours and the Relative Humidity would spike close to 70%. I mistakenly rationalized that since only the temperature had changed and not the actual water concentration in the air there was no impact to samples being handled at the time. I also thought that the lower temperature should actually be better for the samples. It turned out the moisture available for uptake in the samples increased as much as 50%. We never confirmed the actual number or repeated our experiment, but we did make sure samples were no longer exposed during those relative humidity spikes.

In conclusion, the closer the concentration of water vapor in your chamber is to the maximum possible concentration for that temperature, the more ferociously the water vapor will try to get into your samples. “Well duh,” you might say, “higher RH is bad, we all knew that.” And you are right, I’m just saying that I thought I understood it better. I thought it really came down to concentration alone no matter what the temperature (within reason). So, we as stabilitarians need to respect the ferocity of water vapor and continue to treat those RH excursions as if they could be a vicious attack.

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