For a number of different reasons, exhaust system work can be among the most dangerous types of work you can do on your car.
Always observe the following precautions:
Support the car extra securely. Not only will you often be working directly under it, but you'll frequently be using a lot of force, such as heavy hammer blows, to dislodge rusted parts. This can cause a car that is improperly supported to shift and possibly fall.
Wear goggles. Exhaust system parts are almost always rusty. Metal chips can be dislodged, even when you're only turning rusted bolts. Attempting to pry pipes apart with a chisel makes chips fly even more frequently.
If you're using a cutting torch, keep it at a great distance from either the fuel tank or lines. Stop what you're doing and feel the temperature of fuel bearing pipes or the tank frequently. Even slight heat can expand or vaporize the fuel, resulting in accumulated vapor or even a liquid leak near your torch.
Watch where your hammer blows fall. You could easily tap a brake or fuel line when you hit an exhaust system part with a blow. Inspect all lines and hoses in the area where you've been working before driving the car.