"they could easily grow lettuce"

There is a constant stream of cars going through some McDonalds drive-thrus. I would worry a bit about the car exhausts somehow getting to the lettuce, soil etc in sufficient concentration to be an issue. I don't know if this would be a problem or not but as a consumer (who doesn't go to McDonalds) it's something that comes to my mind and I'm sure others (regardless of the type of, um, food they are eating) might also be skived by it.