[Alan T]

Ok found the email from the BAA:

Learn how to prepare for the potential weather conditions

Know your limits.
Heat and humidity increase the physical challenge of running, and health problems can occur when you push beyond what your body can handle. Do not aim for a personal best on a warm, sticky day, particularly if you are not used to such conditions. While running, the body temperature is regulated by the process of sweat evaporating off of the skin. If the humidity in the air is so high that it prevents the process of evaporation of sweat from the skin, you can quickly overheat and literally cook your insides from an elevated body temperature. Check the weather forecast to determine heat and humidity levels. Understand the faster you run, the higher your body core temperature rises.

Acclimate.
It takes 10 days to two weeks for the body to acclimate to keeping cool at higher temperatures. Give your body time to adjust.

Know the signs of heat problems.
If you feel faint, dizzy, disoriented, or your skin is clammy and abnormally hot or cold, slow down or stop running. If symptoms continue, sit or lie down in the shade and seek medical help. Heatstroke occurs when the body fails to regulate its own temperature, and the body temperature continues to rise. Symptoms of heatstroke include mental changes (such as confusion, delirium, or unconsciousness) and skin that is red, hot, and dry, even under the armpits. Heatstroke is a life-threatening medical emergency, requiring emergency medical treatment.

Preexisting Medical Concerns.
If you have heart or respiratory problems or you are on any medications, consult your doctor about running in the heat. If you are feeling ill on race day and experiencing medical problems, you are strongly urged to withdraw from the race.
In some cases it may be in your best interests to run another day. If you have a history of heatstroke/illness, run with extreme caution - your risk of experiencing another bout of heat illness increases.

Drink enough.
Drink throughout the day, so that your urine remains plentiful and pale yellow. Even mild dehydration (scant, dark-yellow urine) will make you feel sluggish and tire early during exercise, and can increase the risk of heat-related problems during exercise. In the heat, sports drinks are even better than water because the sugar and salt they contain form an "active pump" that transports fluid to cells more quickly than water alone. Before workouts or races lasting longer than one hour in the heat, drink 16 ounces of fluid several hours in advance, another 16 ounces in the hour before, and more just before the start if your urine isn't pale.

Don't drink too much.
Overhydrating before and during exercise can cause a dangerous condition known as hyponatremia (water intoxication). This drop in the body's sodium levels can cause nausea, fatigue, vomiting, weakness, and in the most severe cases, seizures, coma, and death. To avoid hyponatremia, do not overdrink, include pretzels or a salted bagel in your pre-run meal, and use a sports drink that contains sodium. During exercise, drink no more than a cup of fluid every 15 to 20 minutes.

Eat a good pre-race meal a few hours before the run.
Try a bagel with peanut butter and a banana—the protein and carbs will fuel your effort and aid in recovery.

Consume a small amount of salt. Eat salted foods like a salt bagel, salted pretzels, or salted nuts. If you’re planning to race, eat salted foods all week prior to the event. On race day, consume one fast-food salt packet at the start line. Have another salt packet halfway through the race.

Protect yourself from the sun. When you are going to be exposed to the intense summer rays of the sun, apply at least 15 SPF sunscreen and wear protective eyewear that filters out UVA and UVB rays. Consider wearing a visor that will shade your eyes and skin but will allow heat to transfer off the top of your head.

Check your meds.
Do not consume products like cold medicines, ephedrine, pseudoephedrine, or anti-diarrhea medicines with dehydrating agents in them. They may increase your risk for heat illness. Caffeine products are only OK in doses you are used to taking on training day. Do not start taking a caffeine product on race day.

Wear light colored synthetic fabrics.
Unlike cotton, synthetics wick moisture from your skin so cooling evaporation can occur. Synthetics also decrease chafing and don't cling and cause a chill. Look for loose-fitting garments with mesh inserts under the arms, on the sides of the torso, down the arms, and on the outer thighs. Acrylic socks keep feet dry and cool.

Age factors.
Young children and teenagers are more susceptible to heat induced injury. Studies have shown that children are at a disadvantage of heating up easily due to their smaller body sizes.