Hi. Welcome to JustAnswer. I am Dr Uzair.I am reviewing your post and will post back with your reply momentarily.
Alright, now this kind of a problem has a few common causes that need to be ruled out first before anything else is done. Number one is gingivitis which is also known as an infection of the gums, this can go unnoticed clinically and the sufferer has no such symptoms, so, this needs to be checked by a physician for any signs of gum disease, the second cause is atrophic rhinitis and this is most common in individuals who have allergies and sinusitis for a long time, in this case the sufferer him/herself does not smell the odor since the receptors are used to it, known as merciful anosmia.
The third common cause is an esophageal diverticulum, which is a pouch like development in the wall of the upper part of the food pipe, this pouch accumulates food which decomposes over time and causes the halitosis (bad breath). You will need to go to your GP first for this and seek the appropriate referral. In case, it is gum disease, a dentist will help, in the second case, an ENT doctor and in the third instance, a gastroenterologist.
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There are certain conditions that can cause that, they are known as porphyrias but all that can be ruled out by blood tests and your GP will make sure all the necessary investigations are carried out.