The shop vac is a Husky 8 gallon/4 horsepower model. Before vacuuming, ensure the correct filter is in it. If you're vacuuming dry material, you'll need the paper filter. If you're vacuuming water, you need the plastic filter. There is a large drain on the front. Ensure that's closed. And if you did vacuum something wet, empty it right away and leave the cover off to let it dry thoroughly. Failure to do so will result in a mouldy, smelly mess.
If you're vacuuming sawdust and the air is suddenly hard to see through and it's difficult to breathe, you've forgotten to put the filter on!