Friday, June 18, 2010

The Grandma and the wet bulb globe temperature

Duluth Tribune News reports
If you weren’t running a marathon along the North Shore of Lake Superior, you probably didn’t notice last June 20 when the temperature rocketed from the 50s into the 70s in the early morning hours.

But if you were in charge of a marathon with thousands of participants, you might have worried that it was too hot even for elite athletes to safely run 26.2 miles.

Despite the heat, last year’s Grandma’s Marathon went the distance without a major problem.

This year, after three straight years of unusually high temperatures on race day, Grandma’s race officials have lowered the temperature at which they will allow the race to start, from a “wet bulb globe temperature’’ of 82, the national marathon standard, to 78.

“That means if the WBGT is 79 or higher, there’s no race,’’ said Scott Keenan, executive director of the Duluth marathon.

“We increased the safety factor at the starting point because we can’t cut a loop off our race, or shorten the course like they can in an out-and-back race. We’re a one-way race and there’s no easy way to get people back,’’ Keenan said. “We not only have racers to worry about, but we don’t want to stress the whole city’s medical system. We don’t want every Gold Cross ambulance in town out on the race course.”

The wet bulb globe temperature is a composite used to estimate the effect of temperature, humidity, wind chill and solar radiation on humans. It’s a fancy name for how it feels for people who are outside. The WBGT is measured at the marathon and half-marathon starting point and at the finish line before and during the race.

If the WBGT hits 90 at any point during the race, the marathon will be stopped. But experts say the best bet is to cancel the race if a 90 reading is possible.

Even in more-usual conditions, “heat-related illness will by far be the majority of patients we see, both along the course and at the finish line,’’ said Dr. Ben Nelson, an SMDC sports medicine physician and medical director of the marathon. “But trying to get people off a course once the race starts is a nightmare. … And some people just won’t stop running. ... That’s why we have more conservative numbers for the start this year. The time to make the call is before the race starts. And I’m confident our system to do that, while it hasn’t been tested, is pretty sound.’’

So far it’s never come close to being that hot or humid at the Duluth race, and no Grandma’s has been canceled. At least two races have been delayed because of stormy weather, but by no more than 20 minutes, Keenan noted. And Grandma’s has never had a fatality due to heat.

In fact, Grandma’s has a national reputation as being a cool marathon where people could run fast.

But the past three years have seen unusually high temperatures, in the 70s at the start and near 80 when many of the racers were finishing. Higher humidity and sunny, calm days can push the WBGT well above the air temperature.

The recent warmer race-day temps appear to be flukes of a wind shift or a short heat-wave. Along the North Shore, a wind shift from east to west can raise temperatures 20 degrees in minutes and counter the lake’s cooling effect.

Usually, though, Lake Superior has a cooling influence for the racers running along its shore — even when it’s hot just a few miles away. For example, on June 17, 1995, Duluth experienced a record high of 93 degrees on race day. But that was at Duluth International Airport on top of the hill.

“The temperature on the race course never got out of the 50s,’’ Keenan noted. “That’s why we usually have been pretty safe from heat issue. ... Even last year, it (the WBGT) never got to that 78 point, let alone to 82, which was the guideline we used last year.’’

Based on Grandma’s start temperatures, it’s more likely that some racers could suffer from temperatures too cold, and sports medicine experts say WGBT temperatures below 50 require special warming stations and should be cause for race officials to consider canceling a marathon.

Heat, however, has made more headlines. Already this year the Stillwater, Minn., marathon on May 30 stopped early when air temperatures climbed above 85, though most of the runners had already finished.

The 2007 the Chicago Marathon, run on what usually is a cool Oct. 6, was stopped at 3.5 hours after more than 50 people were hospitalized due to extreme heat. One runner died. There were more than 300 calls to 911 due to runner health issues, and more than 10,000 starters did not finish. The air temperature hit 88 during the race. The WBGT at the start of the race was 82, right at the WBGT cutoff line suggested by the American College of Sports Medicine.

But a 2008 report by University of Minnesota Medical School Prof. William Roberts, a St. Paul orthopedic physician and medical director of the Twin Cities Marathon, suggests an even more conservative limit. Roberts’ study found that marathons probably shouldn’t be started if the WBGT is above 69. Roberts looked at eight marathons nationwide that were forced to stop or saw major runner health issues due to heat, although most of his findings are based on Twin Cities marathons which now won’t start if it’s warmer than 69 on the WGBT scale.

“This may apply more to fall marathons where people may have lost their acclimation to heat,’’ Roberts said. “I think every race needs to find their own (heat) threshold.’’

The report came after the 2007 Twin Cities marathon was shut down by heat on the same unseasonably warm October day as the Chicago marathon.

“We temporarily closed six hospitals’’ with the overload of racers who had heat problems, Roberts said. “The elite runners usually know when to stop, especially if they are out of the money. A DNF (did not finish) is a lot better than not being able to run for six months. But a regular person may not have that governor to know when to quit. And you can cause some pretty serious problems to your body.’’

mzungo says: this year's race has predicted highs in the mid 60s. Let's hope the weathermen are right!