The news yesterday of the explosions at the finish line of the Boston Marathon shook me to the core. For most of us distance runners, the goal of running Boston is one that gleams in front of us forever: it will probably never happen, but oh, to imagine if it could. Many people consider it the most important, the most meaningful and joyful run of their careers. I had several friends running yesterday, and as I cheered them on remotely, inside I seethed with jealousy. It was horrifying, later that day, to see so much happiness and triumph turned in to so much tragedy. To the running community, it's a punch in the gut. My thoughts are with all the runners, their families, and all Bostonians today.

Many of us saw the events unfolding from the Boston Marathon yesterday, streaming down our Twitter feeds or Facebook feeds in almost-real time. I watched my twitter stream with horror, and amid all the pictures, numbers, evacuations, sorrow, and fear, there was one side note that caught my attention.

It must have been hard to divert all those thousands of runners, and harder to get them evacuated somewhere safe. That takes time, especially when the diversion is unplanned, and after a run like a marathon, time spent outside can make hypothermia a real possibility. I hope that as many people as possible got back safe, sheltered, and warm with their loved ones.

It might seem odd to non-runners that runners would be out and suffering from hypothermia. After all, they are running. You don't stay cold for long. But in fact, the sweating and work that we are doing makes hypothermia that much more of a possibility, and explains why, when we cross the finish, we are often quickly enveloped in weird silvery "space blankets" to help keep the heat in.

Hypothermia is when your core body temperature drops below the required norm (37 degrees Celsius or 98.6 degrees Fahrenheit) for normal bodily function. Very mild hypothermia is typical and seen very often, in the form of shivering and vasoconstriction (when your fingernail beds turn blue). When it gets more severe, you begin to lose muscle coordination, and slurred words and stumbling may happen, along with confusion. In sever hypothermia, the exposed skin is blue, words are very slurred, and people begin to suffer severe confusion and irrational behavior, as well as sometimes burrowing into or under things. In cases of even severe hypothermia, warming someone up and rehydrating them (hypothermia induces diuresis, or loss of water, which means urination) will restore them to normal fairly quickly.

Historically, hypothermia was a condition of the poor (remember the Poor Little Match Girl?), particularly the elderly, and also afflicted soldiers (particularly those who attempted things like land wars in Asia). But now, in the U.S., hypothermia is also common among people who perform in extreme outdoor sports, such as mountain climbing and, sometimes, distance running.

I myself have suffered many a bout of mild hypothermia (and a few cases of moderate hypothermia) in my life as a distance runner. It happens to some runners even if they don't spend a lot of time outdoors standing still. Why?

As I explained in this post, when you begin to run, you're cold. As you run, your core body temperature rises, you are producing heat. But your OUTSIDE is still cold. You can only lose the heat you've built up in your core when the heat reaches the skin and begins to dissipate. At that point, you get hot and begin to sweat. All is well, you are losing the heat you need to lose.

But when you stop, the situation changes. If it's chilly out (and the high today in Boston was 52 F, so chilly if you're standing around), you need to retain heat. But at first, your core temperature is still elevated, and you continue to lose heat, trying to restore the balance. In a warm environment, you'd just go back to normal, but a colder environment means the heat loss is more meaningful. Soon, you've lost too much heat, and cold begins to set in. If you continue to lose heat, eventually, you will end up hypothermic.

There are several reasons that runners can be at particularly at risk:

1. We aren't dressed to stand around in the cold. Usually runners dress for weather that is a good 20 degrees F above what the temperature actually is. You don't want to carry around sweatshirts and pants for a 26.2 mile run. Many of the runners out today were wearing shorts and t shirts (or, for the faster runners, even less), and left the rest of their gear to pick up after the race. But when you can't get to that gear, you can't get warm.

2. We are hypoglycemic and dehydrated. This means that our blood glucose is low (as you might expect after you've run almost 26.2 miles), and we are low on fluids. Many long distance runners take gels or blocks of concentrated sugars to keep blood sugar higher, and the vast majority are very careful to stay hydrated. But a marathon is a serious test of physical endurance, and you can only make up for so much. At the end of the race, you will be both hypoglycemic and dehydrated. Both of these conditions can exacerbate symptoms of hypothermia.

3. We are small. Many runners are relatively lightweight, with little body fat. You need to be mostly muscle and bone to run 26.2 in the time required to qualify for Boston. This means body mass is low, and heat dissipation can happen more quickly than it might in a larger person. The excessive heat loss compared to size means more of a risk for hypothermia.

All of these risk factors mean that when runners were diverted and stopped before the finish, they were stuck in not enough clothing, with not enough water, in chilly weather. And so there were probably many cases of mild to moderate hypothermia.

Luckily, and as we saw yesterday, runners, and people in general, are a wonderful bunch. Many people ran towards the explosions instead of away from them, and I am sure that all those diverted were helping each other in whatever way they could. Many of the people in Back Bay opened up their homes to cold runners until they could rejoin their friends and families. The events at the Boston Marathon were terrible and heartbreaking, but they also showed the wonderful strength and skills of the first responders, the bystanders, and the runners themselves.

(From Facebook)