How do I get my parents to stop smoking?

My parents have been smoking for years. I have asthma and I have a 3 year old little brother. My parents try not to smoke around us and used to smoke out of our chimney in the winter. But now we just moved into a new house, and there is no fireplace so they just smoke in the house. They can’t afford to use those quit smoking programs and buying cigarettes 3 times a day doesn’t help. They are truly amazing people, but smoking isn’t really making a good influence on others especially my brother and I. How can I get them to stop smoking?