According to the World Health Organization, there are about one billion smokers in the world.
Between five and six million people die each year from smoking-related diseases and illnesses, which means it's important that doctors study the effects of smoking. One method researchers have used to study smoking over the years is placing rats in boxes full of smoke and analyzing their lungs after different levels of exposure.
Animal rights organizations have made a lot of progress in recent years, and researchers are being pressured to find a better way to test their hypotheses. In this case, they did, and it's amazing.