Scientists have long known that smoking damages the lungs, but a new study has now shown in detail how smoking physically changes lung tissue itself.
Researchers at University of California, Riverside have directly measured how smoking affects the mechanical behavior of human lungs.
Their findings, published in the Journal of the Royal Society Interface, reveal that smoking makes lung tissue much stiffer, similar to the damage seen in lung fibrosis, a serious disease that causes scarring in the lungs.
The study focused on lung parenchyma, the soft, sponge-like tissue that makes up most of the lungs and helps people breathe.
To carry out the research, scientists used donated human lungs from both smokers and non-smokers. They carefully removed tiny square pieces of lung tissue and stretched them while measuring how strongly the tissue resisted being pulled.
The results showed a major difference between healthy lungs and lungs from smokers.
When the tissue from smokers was stretched, it became much stiffer and resisted expansion more strongly than healthy tissue. This stiffening effect resembles fibrosis, where scar tissue builds up and makes breathing increasingly difficult over time.
According to the researchers, this is the first time scientists have directly measured these changes in human lung tissue itself.
Most earlier studies relied on animal experiments or stretched tissue in only one direction. However, real lungs expand in many directions at once every time a person breathes.
To better copy natural breathing, the research team stretched the tissue across several directions simultaneously. This gave a more realistic picture of how lungs behave inside the body.
The scientists also discovered that lungs are not mechanically uniform. Tissue from the upper parts of the lungs tended to be stiffer than tissue from the lower regions, even within the same lung.
The researchers think gravity may partly explain this difference. Since humans spend most of their lives standing upright, the upper lungs experience different physical forces over many years compared to the lower lungs.
These uneven mechanical properties could help explain why some lung injuries affect certain areas more than others.
For example, patients on ventilators sometimes develop damage in specific regions of the lungs rather than evenly throughout the organ. Some lung areas may simply be more vulnerable to overstretching.
The study also found important differences between human lungs and animal lungs. Human tissue lost more energy during repeated stretching compared to mouse lungs, suggesting animal studies may not always accurately reflect how human lungs work.
This finding matters because scientists are increasingly building computer-generated “digital twin” lungs to simulate breathing, disease, and medical treatments. If those computer models are based mostly on animal data, they may miss important features of real human lungs.
The researchers also noticed early signs that lungs may naturally become stiffer with age, though they say more studies are needed to confirm this.
Because donated human lungs suitable for testing are rare, gathering enough samples remains challenging.
Still, the scientists believe their work could eventually improve ventilators, surgical planning, and computer models used to predict how damaged lungs respond to stress.
The researchers say understanding the true mechanics of human lungs is essential if future medical technology is going to accurately support the way people actually breathe.
