Smoking is bad for you, as is second-hand smoking. It is the leading cause of preventable deaths in the United States, causing more than 480,000 deaths annually. Just something we should probably get out of the way before we tell you the next bit: Before it was banned on airplanes, it may have actually saved passengers' lives.

In 1988, smoking was finally banned on all planes in the US. Not everyone was happy about it – specifically smokers and plane mechanics. While smokers were stuck with the inconvenience of not being able to light up 35,000 feet (10.6 kilometers) in the air, mechanics lost a way to spot potentially fatal flaws in the plane. It turns out that aircraft mechanics would use stains from nicotine to spot cracks in the aircraft.

As the planes ascended and the passenger cabin pressurized, air could escape through any tiny cracks in the aircraft. If the air being blown through the hole happened to contain staining chemicals such as nicotine and tar from cigarette smoke, stains would build up around the cracks. Mechanics could then see the stain and know that there was a crack in need of repair. 

This wasn't just a hypothetical: an infamous crash once occurred because mechanics missed these telltale signs. On May 25, 2002, China Airlines Flight 611 broke up mid-air, killing all 225 people on board. The crash was caused by a botched repair to the aircraft some 22 years earlier.

"One piece of evidence of the metal fatigue is contained in pictures that were taken during a routine inspection of the plane years before the crash. The photos showed visible brown nicotine stains around the doubler plate," aviation historian Martin W Bowman wrote in his book Boeing 747: A History.

"This nicotine was deposited by smoke from the cigarettes of people who were smoking about seven years before the disaster (smoking was allowed in a pressurized plane at that time). The doubler plate had a brown nicotine stain all the way around it that could have been detected visually by any of the engineers when they inspected the plane. The stain would have suggested that there might be a crack caused by metal fatigue behind the doubler plate, as the nicotine slowly seeped out due to pressure that built up when the plane reached its cruising altitude."

"The stains were apparently not noticed and no correction was made to the doubler plate, which caused the crash to happen."

Since the smoking bans on airplanes around the world, other methods of detecting cracks have been developed. Routine inspections of older planes were introduced by the Federal Aviation Administration following a separate incident caused by a crack, which killed one flight attendant.

Modern-day planes still use visual inspections but also have high-tech sensors at their disposal to detect any cracks, which is much more sophisticated than looking for a brown smudge – and far better for the lungs of the cabin crew.