B

Reduced salivary flow in xerostomia means less bicarbonate and other buffering ions, so the resting saliva becomes more acidic. Clinical studies show that patients with radiation-induced or sleep-apnoea-related xerostomia have salivary pH values around 5.9–6.4, significantly below the normal neutral range, confirming a low pH state [Sim, 2018, PMID 29569726; Makeeva, 2021, PMID 33548090].