Slides measuring how much pollution is in the air during the last couple days show it's much worse than normal and could be a problem for the athletes.
"The air we are breathing is so poorly and the oxygen content is so low that they will have problems with achey, muscle pains," said Dr. Joseph Leija, allergist, Loyola Gottlieb Hospital.
The dirty air combined with warmer than normal temperatures are just a few additional challenges for many of the runners from Team World Vision. The charity works to help children and bring clean water to communities in Africa. Four of them are raising more money by running 100 miles starting overnight Saturday and finishing with the marathon Sunday. They say their challenge is nothing compared to what most Kenyans face.
"People are always ill, dying at very young ages. The children can't go to school. They are walking miles to get the same dirty water," said Hannah Covert, Team World Vision.
"We have each committed to is finding sponsors for 100 children," said Michael Chitwood, Team World Vision.
Team World Vision gathered for a pre-race meal Friday night with some of the elite Kenyan runners who appreciate all the work the charity does for people in their country.
"What they are doing is changing people's lives, and what they are doing is giving people an opportunity to become a better person in their life," said Kenyan runner Wesley Korir.
With temperatures expected to approach 80 on race day, organizers are adding water stations and medical personnel. Many runners aren't worried.
"I'm like a camel. I love that kind of weather. I don't like the cold. So the real runners like cooler weather, but this is tailor-made for me," Frank Fragale, marathon runner, said.
"Whatever your personal goals are you want to take a personal check, pay attention to conditions, pay attention to the messaging," Carey Pinkowski, director, Bank of America Chicago Marathon, said.
Weather will likely not be a factor for the elite runners, who are expected to finish before temperatures peak. Russian Liliya Shobukhova is vying for her third straight Chicago marathon win. Wheel chair defending champion Amanda McGrory of Champaign is going for win number four.
"I've got a lot of friends in this city, so I think it'll be a fun one. And you know, the more pressure just makes you a little more proud," McGrory said.
On Friday, race organizers honored Sammy Wanjiru, the winner of the past two men's races in Chicago. He died last May in a fall at his home in Africa. Wanjiru's absence could help the chances of fellow Kenyan Moses Mosop and American Ryan Hall, who's working toward another Olympics.
"This race is huge for me in terms of my progression to the Olympics, practicing and trying to get on the podium. For me, all these next marathons are all about finishing in the top three," Hall said.
The marathon has long been held on Columbus Day weekend. But this year it coincides with Yom Kippur, the Jewish Day of Atonement, which calls for more than 24 hours of fasting between Friday and Saturday evenings. For many Jewish runners, the holiday is forcing a change in their pre-race ritual of loading up on carbs.
"I've been drinking a lot of water today. And this whole week I've just been drinking and really watching exactly what I eat," Samantha Spolter, marathon participant, said.
"It is not about 24 hours but the day before, the night before, to have a baseline for Sunday morning," said Dr. George Chiampas, medical director at marathon, said.
Last year's race added an estimated $170 million to the local economy. With 45,000 men and woman expected to run on Sunday, this year's race could have a similar impact.