Did you think that cigarettes are made of paper and cotton and must, therefore, be biodegradable? It is a common misconception.
The reality is that 98% of cigarette filters are made of plastic fibers (cellulose acetate) and contain hundreds of toxic chemicals, including nicotine, lead, and arsenic. Cigarette butts are the single most collected item in beach cleanups (up to 1/3 of all collected items) and it is estimated that there are about 4.5 trillion individual butts contaminating the world. These filters can take up to 10 years to fully degrade, and during that time they release contaminants that remain in the environment even longer. Filters can also get eaten by terrestrial and aquatic animals. And they are extremely difficult to remove from the environment because they are so small and require a lot of labor.
All in all, not even counting the health problems they cause in humans who smoke or are exposed to second-hand smoke, cigarettes are an environmental problem. If you smoke, you are not only making yourself sick; you are also contributing to making the world sick, even if you do dispose of your cigarette butts properly, because you are funding the industry that is causing all this pollution.