You might already know this but you should have at least 24 hours' notice prior to a viewing, and if there's inadequate space in the property the agent shouldn't be bringing in too many people (including him/herself) at any one time. You should tell them this if they're not already playing ball. No visitors should be touching anything and all visitors, including the agent should be equipped with hand gel, masks and gloves. If the agent and or visitors don't adhere to the TPO guidance then cease their activities and complain to the relevant parties in writing (that's the landlord, letting and or selling agent). Also, if you feel at all ill then no one should be coming in whatsoever. And I'd advise your vacating just for the few minutes in which a viewing should take place, and asking the agent to provide the relevant supplies for cleaning etc.