Health and safety at work

Hello everyone, I have a health and safety issue at work. I am a community carer, both my midwife and I have outlined to my employer that they are not to put me in any environment with second hand smoke relating to my employment as it is a health and safety issue to myself and my unborn baby. They have this in writing aswell as being telephoned. Today I’ve had to argue with them that I’m not to go to houses that are being smoked in as yet again they’ve tried to send me there. The client in question is paraplegic and remains bed bound and therefore smokes in the property being unable to go outdoors. Despite the fact of the risk of second hand smoke they stated THEY were happy to send me there just because the house wasn’t overly smokey but still wasn’t the point as the room we work in is the room she smokes in and just because it doesn’t smell doesn’t mean the smoke isn’t there. I had to argue to get huffed at and get the response of “I don’t get what the problem is but fine I’ll change it. It limits where we can put you” and I said “And? My midwife has even sent you a letter saying that it’s a health and safety hazard a month or 2 ago. I phoned on Wednesday and now I’m phoning today because you didn’t swap my shift to a more suitable one so it’s not like you haven’t had sufficient warning or notice”. I’m now having to try and get back in touch with my midwife because even though they have changed the shift, I am a walker and I’m working on a 2 carer shift with a carer who drives and smokes in her car and I’m unable to just walk there because some of the calls are remote. They don’t seem to be removing the risk and it’s so frustrating and I get faced with an argument. It’s unnecessary stress. Anyone know how to take this further?