My college xc team does probably 80% of our mileage on bike paths or sidewalks right next to roads. Our state also hss worse than average air pollution. Is it a big deal? I've heard running next to roads is bad for your lungs.
Of course, it's better to run in a park zone or not too close to a road at least. When you are running, your lungs work very efficiently. You inhale deeper, so it's more dangerous than you just walk next to roads. Runners may get asthma or other diseases. It'd be nice to change your route for saving your health.
I coach in a similar situation. Honestly, I worry more about someone getting hit than I do about fumes.
I don't think you are going to have any long term problems unless it is real smoggy where you are at. Obviously parks and other runs are better if at all possible, but it may not be in your area.
This is a fairly common situation for schools in medium sized cities (like the mid-west, the south or Texas) in non health conscious areas. Big cities typically have parks and green belts, small towns have rural dirt roads.