How much do boston terriers smell?

Boston's don't smell any more than any other dog if they're kept clean. They do not have gas unless they are fed something to give it to them. On a good quaity grain-free dog food, they should be fine.
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My parents had a Boston Terrier for a while, and it didn't smell at all. It was actually a very clean tidy little dog. Great pet.
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You can't predict passing gas.

They can smell from infections, skin problems , ears, eyes etc. I think their faces tend to smell cuz of the smushed look with everything being so close together but their body doesn't have much of an odor.

Perhaps it is anal glands you are smelling & not gas. This site has all the info on glands if you think it might be glands instead.

http://www.marvistavet.com/html/body_anal_sacs.html
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groomer

For the past 30 years I had Boston Terriers until recently. Typically the smell you are experiencing is their anal glands - if you take them to the vet and have their glands expressed they will be much more comfortable and so will you. Do not let a groomer convince you that they can express the glands because in most cases a groomer will only express them externally rather than internally.They are great family members and If you already have one or are thinking of getting one you will be so very happy.All my best. Granmatictac
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past terrier owner

Same odor as any other dog. Nothing more, nothing less .. as long as properly groomed. The gas will depend on their diets. A healthy diet and this won't be a problem either.
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Todays Pet - Tarantulas and You!
http://todayspet.blogspot

Food can be part of the equation…if you are not already feeding a high quality holistic pet food, I highly recommend you try - many, many problems are related to diet.