The environment can wreak havoc on performance this time of year. The combination of heat and pollen can really throw your system out of whack. The cardiovascular, digestive, and respiratory systems have to go into overdrive to handle the different stimulus. It's critical that you work to mitigate the factors before, during, and after exercise.
When it's really hot outside, your body diverts blood and water from the stomach to the skin in an effort cool the body. Beyond simply dehydrating you, it makes it more difficult to digest anything in your stomach. Often you'll see much higher heart rates than normal, also.
Some simple ways to attack the heat:
Lots of small sips of water with electrolytes starting [at least] the day before working out in the heat.
Don't bombard your system with lots of food at once.
Avoid overloading with sugars leading into the event.
Keep cool as long as you can. Maximizing your use of air conditioning, shade, ice, and even tools like ice socks (nylons filled with ice, and placed on your upper back or in jersey pockets).
Make ice bottles. Fill up your water bottles half way before an event, and freeze them on their side in the freezer. Fill them the rest of the way with sports drink the morning of, and transport them in a cooler. Having the solid block of ice makes the ice last longer.
In addition to the heat, pollen counts have been on the rise consistently for the last 20 years. Longer, and more intense pollen seasons are a reality we're unlikely to be able to avoid. Itchy eyes, a runny nose, sore throat, and wheezing are all common reactions to environmental allergens. Post nasal drip can even cause an upset stomach in the morning because of draining that happens when you're asleep.
There are a number of strategies you can employ to combat allergies:
See your doctor to try to determine what you're allergic to.
Try over-the-counter antihistamine medications like Zyrtec or Claritin.
Keep the windows closed in your home.
Change clothes when you get home to limit your pollen exposure.
Shower before bed.
Wash your sheets more frequently.
Consider other common home remedies (like neti pots and eating local honey).
Athletes work really hard for months and months to prime their bodies for performance, only to be attacked by the environment once the season begins. While the suggestions above may only have a small impact, they are all relatively easy to accomplish and will add up to a significant advantage over your competitors.