Volatile Sulfur Compounds (VSC) like hydrogen sulphide are produced by gum disease causing bacteria and are a cause of bad mouth odour. 

According to a study by Morita, sulfur levels were higher in patients with more severe gum disease. If you have persistent bad mouth odour, this may be a sign of active gum disease. 

more details here:

Morita M, Wang HL. Relationship of sulcular sulfide level to severity of periodontal disease and BANA test. J Periodontol. 2001 Jan;72(1):74-8. doi: 10.1902/jop.2001.72.1.74. PMID: 11210076.

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