I drive tractor trailer for JB Hunt. Over the last 4 Years, the three trucks I have been assigned to have had exhaust leaks that vented into the cab. One was a Freightliner (approx 2017 model), and two have been Internationals. These exhaust leaks caused me medical problems, requiring that I take off time unscheduled. The last three trucks I have been assigned to have had exhaust leaks into the cab. Some leaks I only found with a thermal camera. The exhaust leaks are not the main problem. The problem is that design of these trucks cause any leaking engine gases to be pushed into the cab of the truck, mostly through the air conditioning/heater vents. I have tested this. I zip tied a can of orange air freshener under the hood. The can was positioned in such a way as to point away from the air intake box, toward the back of the engine. When driving at highway speed, the orange air freshener was very strong coming into the cab from the a/c vents. The truck is supposed to be getting the fresh air from outside the truck, and forward of where the air freshener was spraying. I originally thought the only problem was that the cabin air intake box is located under the hood, in immediate proximity to all of the leaks. This in itself is a poor design. Even with duct tape over the openings into the cabin air box, any gases (or air freshener) is immediately pulled into the cab through the a/c and heat. I have no idea how this occurs. The design of the trucks allows any leaki