To be honest, I’m not too sure myself. This was the best I could find online:
Zinc-air batteries are usually referred as mercury free, when, in fact, some have mercury in their constitution. For example, icellTech states that they use less than 25 mg of mercury per battery (icellTech, 2004) while Duracell (2008) reports that their batteries satisfy the limit of 0.1 mg/m3. Zhang, Bruce, and Zhang (2011) state that despite the benefits of reduced mercury in zinc-air batteries, there are still some disadvantages such as limitation of power (low voltage and current), evaporation of the electrolyte (failure due to the entry of air into the battery), adverse reactions arising from the presence of other gases in the environment (e.g., the entry of carbon dioxide can produce solid carbide), and production of solid elements (difficult and costly to dispose of). These authors suggest that lithium-air technology may replace zinc-air batteries in the near future
Thieme E-Journals - International Archives of Otorhinolaryngology / Full Text10
Thieme E-Books & E-Journals
Obviously dated info too. I had trouble finding any recent coverage of mercury free hearing aid batteries in the journals.