husband is a smoker.

My husband is a smoker. He is a VERY HEAVY SMOKER. But, my house is a smoking free zone. I don't allow him to smoke in the house/toilet or anywhere near my house. I'm currently 28 weeks pregnant. I tend to be more breathless when i smell smoke if he is smoking in the corridor or anywhere near the house. How do i get him to understand that the smoke can actually harm my kids and myself?