It is wrong that cigarette filters cannot filter out harmful substances such as CO; There are many poisons in cigarettes, nicotine is only one of them, and B is wrong; Smoking is harmful to oneself, harmful substances in cigarette smoke, through the breathing of passive smokers, cause harm to passive smokers, D wrong; CO is easy to combine with hemoglobin, which reduces the oxygen permeability of hemoglobin. The human body is poisoned by lack of oxygen. C is correct.