Sensitive subject with my mom about smelling like smoke..

Okay, my mom and her husband still smoke. They smoke in their house. They're so old school though, that they believe that if they're not smoking WHILE my kids or someone is there, that the house doesn't stink. It's gotten really really bad the past couple of years. Now, I can't even walk in the house at all. I am so thankful that when she picks my kids up twice a week for me that they're playing outside when I get there and I don't have to go in. The problem is this - my boys go in there and drop their book bags and stuff and either stay in there for a snack or to play or whatever, read, watch tv. They're only there for about an hour or so. When I pick them up and they get in my car, I gag because the smell is so bad. I don't want my kids around it! I know many of you will say to just tell her but you have to understand my mom. She'll take extreme offense to this. It will be a big blow-out and she may not speak to me for a period of time. I just don't know what to do. She knows I quit and is so proud of me, she's so supportive. Her and her dh are waiting on e-cigs to come and then they're "going to quit smoking". I don't know if I believe it and will when I see it. I hope so badly that something gives and they do quit. She has COPD and her husband thought last year that he had lung cancer. He did end up having several spots on his lungs but they weren't cancerous. So sad and scary. I just wish there was an easy way to tell her this. Example,quit smoking now, she once gave us some tshirts that had been hanging in her closet that she didn't want anymore. She said, well, they have those yellow marks on the shoulders from hanging on the hanger. I told her that was from smoke. She said, it couldn't be because they didn't smoke in the bedroom! I just let it go and said,smoking cessation, whatever, I've never had yellow marks on shirts from just hanging in the closet. We never smoked in our house when I did smoke. We (dh still goes) went to the garage or outside every single time. Ugh. Thanks for letting me vent this. Any advice on how to approach her? I kind of have to walk on egg-shells around my mom sometimes. You just never know what will set her off.