He stresses: “There’s no cure for popcorn lung. Once the lungs are damaged, treatment is limited to managing symptoms. This can include bronchodilators, steroids, and in extreme cases, lung transplantation. For this reason, prevention – not treatment – is the best and only defence.”

Diacetyl is not the only chemical of concern. The damage caused by inhaling it is typically irreversible and often results in disabling respiratory harm.

Though diacetyl has been banned in e-cigarettes across the EU and UK, it can still appear in unregulated or illegal products—and it remains present in some products in the US and other parts of the world.

Worryingly, other toxic substances may also trigger popcorn lung.

“Volatile carbonyls like formaldehyde and acetaldehyde – both of which have also been detected in e-cigarette vapours – are among them,” O’Shea notes.

Experts estimate there are currently more than 180 flavouring agents in use in vaping products, many of which, when heated, break down into new substances that haven’t been evaluated for their effects on the lungs.

“Because vapers’ lungs are exposed to so many chemicals, it’s not possible to directly prove that diacetyl is the cause of the disease in any specific case,” O’Shea writes. “But that doesn’t negate the proven risks of inhaling it. Even if diacetyl isn’t the sole culprit, cumulative exposure to multiple chemicals and their byproducts could increase the risk of popcorn lung and other respiratory conditions.”

Given the irreversible nature of the harm and the number of unknowns still surrounding vaping chemicals, Professor O’Shea concludes that preventing exposure is the only effective path forward.