By now, most people know that smoking can cause a number of illnesses, such as lung cancer and heart disease. One teacher in Hong Kong, Ryan Au, was so concerned that his students were starting to smoke, so he set up a demonstration showing the effects of what smoking cigarettes would do to a set of pig lungs. Au is a teacher at Yan Chai Hospital Tung Chi Ying Memorial School in Hong Kong.

The demonstration shows two sets of healthy pigs’ lungs. Then, one of the lungs is inflated with clean air and the other with smoke that would equal 60 cigarettes. At the end of the experiment, Au cuts open the trachea of both the lungs. What he reveals is that the one without the smoke is perfectly pink and the other smoke-filled one is black and charred in appearance.

Of course, smoking affects the color of lungs, but what’s shocking to see is how quickly this happens only after 60 cigarettes. Many chain smokers smoke about a pack of cigarettes per day — that’s 20 cigarettes. So over the course of three days, this is how a person’s lungs might look.

According to the Action On Smoking & Health, there have been over one billion tobacco-related deaths worldwide, and that number is supposed to increase to 1.6 billion over the next two decades.

Despite efforts of many public health organizations to warn people of the effects of smoking, it’s still a growing and problematic epidemic.