A fatigued woman is sweating on the floor in front of a couch while waves of humidity flow across room.

When Humidity Triggers MS Symptoms

I was raised in the Pacific Northwest, where summer temperatures rarely soar into the 90s. A classic hot day here hovers in the 80s with low humidity, in stark contrast to damp, rainy, foggy, misty autumns, winters, and springs. When it gets hot, I tend to notice. In the Seattle area, so much also depends upon the cloud layer above, which is, in so many ways, more oppressive than the summer sun.

Imagine my surprise, then, when I realized I didn’t mind being out in the bright, hot sun at nearly 120°F on a recent trip to Arizona. Meanwhile, when I lived in Chicago, I suffered through heat waves hovering around the 100°F mark—with 100 percent humidity — after sticky fingers of humidity visited us all the way from the Gulf of Mexico.

Humidity and MS

Thinking about this makes me realize that my personal MS trigger isn’t heat, but humidity.

“But it’s a dry heat…” Yeah, yeah...I’ve heard all the jokes. But for me, the heat index — a meteorological formula that calculates how hot it really feels based on both heat and humidity — predicts with more accuracy how I’ll feel when the sun comes out.

I decided to examine humidity as an MS foil.

Yep, humidity counts

The National MS Society definitely includes humidity in its discussion about heat and temperature sensitivity, offering this explanation:
“These temporary changes can result from even a slight elevation in core body temperature (one-quarter to one-half of a degree). An elevated temperature further impairs the ability of a demyelinated nerve to conduct electrical impulses.”1

Well, that certainly makes sense to me. When I feel warm and damp at the same time, I fall into a sluggish fog, and everything slows down, including my movements and my speech. This feeling almost always precedes an MS hug. I might also face the onset of a migraine, digestive issues, and overwhelming fatigue.

What about Uhthoff’s Phenomenon?

You may have heard of the Hot Bath Test. Doctors tried these to identify people with MS in the 1950s, well before the MRI could take snapshots of our brains.2

What the test really did was trigger something now known as Uhthoff's Phenomenon (UP), something I have more than a passing acquaintance with. UP describes temporary worsening of neurological function in people with MS caused by elevated core temperature. It lasts less than 24 hours, but that doesn’t mean it doesn’t leave you feeling like a bag of wet cement.

More recently, a sauna test performed on people with MS sought to measure the effect of both ambient temperature and relative humidity on the subjects' symptoms. They confirmed that, in higher humidity situations, people with MS are more likely to experience elevated core body temperature compared to healthy controls.3

Does hydration have an impact?

I realize that, had I felt any symptoms while in Arizona during that super-spicy week, they had to have been minor. However, I do wonder about a connection to the increase in water I drank while in AZ, versus in places where the heat index swells mostly because of elevated humidity.

Similarly, I’m reminded of many trips we’ve taken to Honolulu. Yes, it’s warm there, and yes, it’s humid, but that constant island breeze and frequent sprinkles of cool rain in the afternoon usually keep my temperature steady. But! It also helps that I favor drinking lilikoi cocktails and hibiscus tea.

It’s in the water

And therein lies the solution for my humidity woes, if I’m paying attention at all. I definitely don’t drink enough water on a daily basis, but I do tend to drink more when I’m in a dry, hot climate because I feel my lips chapping and am reminded to hydrate.

But when it’s humid, I don’t chase heat with water. No wonder brain fog and fatigue result! I’m still sweating through my clothes, but I don’t replenish my fluids like I do when I’m in the desert.

Not surprisingly, a study published in 2016 basically confirmed this hypothesis, concluding that “hydration status correlates with self-reported fatigue, with lower fatigue scores found in those with HiH [high-hydration] status.”4

An easy fix

So yes, for me, it’s not the heat, it’s the humidity. But it’s also my lack of vigilance in maintaining proper hydration. I call that an easy fix, considering I still have too many water bottles on hand, even after the move.

I hope this simple revelation helps you the next time you face a period of hot weather with air so swamped with moisture you can part it with your finger.

Drink up!

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