The comment, tweeted by Dr. Brittany Bankhead-Kendall, a trauma surgeon at Texas Tech University Health Sciences Center, went viral last year, making headlines around the globe for exposing the havoc the new virus can wreak on the lungs.

COVID lungs “collapse. And they clot off. And the shortness of breath lingers on … and on … and on,” she tweeted.

Americans have long trashed their lungs via smoking, with catastrophic results. Cigarette smoking is behind one in five deaths in the U.S., according to the U.S. Centers for Disease Control and Prevention. Between 2005 and 2009, smoking caused nearly a half million deaths a year, though only 60,000 or so annually were directly attributable to respiratory ailments.

In contrast, COVID caused more than 350,000 fatalities in 2020, making it the No. 3 cause of death in the U.S.—and likely contributing to deaths in other categories, like heart disease, the No. 1 killer, and diabetes, the No. 58 killer, among others.

Like smoking, COVID-19 can cause lung damage and even kill. It can do so in several ways, from causing pneumonia and acute respiratory distress syndrome, or ARDS—a form of lung failure—to sepsis.

Pick your poison

Which is worse for your lungs?

“They’re both really bad—but bad in different ways,” Dr. Panagis Galiatsatos, an assistant professor in Johns Hopkins’s division of pulmonary and critical care medicine, tells Fortune. He treats long COVID patients in an outpatient setting, as well as critically ill patients in the ICU, and runs the university’s Tobacco Treatment Clinic.

“Smoking, no pun intended, burns the lungs,” Galiatsatos says. But “it’s a slow burn.”

“People who experience smoking-related complications experience them after two or three decades of doing that activity, and they’ve suddenly lost their lung function.”

COVID comes on much quicker and is “hard on the lungs, horrible,” he says.

But there’s good news for those with even severe COVID: The damage, in all likelihood, isn’t permanent.

“If you had good, healthy behaviors to begin with, even with a lung scar or two, you can probably still overcome that damage,” Galiatsatos says, though it could take a year or two of treatment under a good clinician.

Once smokers start to develop symptoms of lung disease, however, much of the damage is likely irreversible, Galiatsatos says.

When it comes to smokers who contract COVID: “Smoking 100% raises your risk of any lung infection having a horrible outcome, horrible consequences,” he says, adding that he sees post-COVID lung complications more frequently in those who smoke or vape than in those who don’t.

COPD ‘isn’t attractive either’

As for Bankhead-Kendall’s tweet about the horrors of post-COVID lungs, “I would gladly point someone at a lung with significant emphysema and say, ‘This isn’t attractive either,’” Galiatsatos says, adding that there’s a risk of “getting lost in the messaging of whose lungs are worse.”

The COVID pandemic has helped to shine a light on the tobacco pandemic, which is underappreciated and often ignored. Tobacco kills as many as 8 million each year, according to the World Health Organization—more than the roughly 6.5 million deaths the pandemic has caused in its nearly three years so far.

Comparing the two, however, is “unfair” because each causes a different type of lung damage, Galiatsatos says.

“They’re both bad—I promise you.”