It may not be. Researchers at Lawrence Berkeley National Laboratory showed that nicotine in tobacco smoke settles on indoor surfaces, such as furniture, carpeting and clothing, where it can combine with nitrous acid commonly emitted by household appliances and automobile exhausts to form two known and potent nitrosamines that cause cancers in humans and animals (Proceedings of the National Academy of Sciences, February 2010).
Air conditioners, fans and open windows cannot remove chemicals from cigarette smoke that have already settled on and been absorbed by walls, rugs and furniture. The carcinogens continue to accumulate over time to become even more toxic. Nobody knows how much harm these chemicals actually cause or how long it takes to get them out of the environment.