Chemicals widely used to flavour e-cigarettes may impair our lungs' first line of defence against dirt and allergens, potentially increasing the risk of diseases such as asthma, a Harvard study has found.
Researchers found that two chemicals, diacetyl and 2,3-pentanedione, affect the function of cilia - antennae-like protuberances that are present on 50-75 per cent of the cells that line human airways.
They play a key role in keeping the human airway clear of mucus and dirt and allow people to breathe easily and without irritation.
Impaired cilia function has been linked with lung diseases such as chronic obstructive pulmonary disease (COPD) and asthma.
"Although chemicals used to flavour e-cigs are frequently used, little has been known about the mechanism of how they impact health," said Quan Lu, an associate professor at the Harvard University in the US.
"Our new study suggests that these chemicals may be harming cilia - the first line of defence in the lungs - by altering gene expression related to cilia production and function," Lu said.
The study, published in the journal Scientific Reports, is the first to look at the impact of flavouring chemicals in human epithelial cells, which are the type that line the lungs. The researchers have previously found that flavouring chemicals - primarily diacetyl and 2,3-pentanedione - are present in over 90 per cent of e-cigarettes tested.
Diacetyl is also used as a flavouring agent in foods such as butter-flavoured microwave popcorn, baked goods, and candy.
While it is considered a safe ingredient in foods, evidence suggests that Diacetyl can be dangerous when inhaled.
It has been previously linked with bronchiolitis obliterans, a debilitating lung disease that was dubbed "popcorn lung" because it first appeared in workers who inhaled artificial butter flavour in microwave popcorn processing facilities.
After the link between diacetyl and popcorn lung was reported, 2,3-pentanedione was sometimes used as a substitute.
In the study, researchers used novel lab techniques that allowed them to examine the impact of both diacetyl and 2,3-pentanedione on epithelial cells in a system that closely mimicked the human airway epithelium in vivo.
They exposed normal human bronchial epithelial (NHBE) cells to the chemicals for 24 hours.
They found that both diacetyl and 2,3-pentanedione were linked with changes in gene expression that could impair both the production and function of cilia.
In addition, the researchers found that even low levels of both chemicals affected gene expression, suggesting that current standards for safe limits of exposure to these chemicals for workers may not be sufficient.
There are no such standards for e-cigarette users, researchers said.
"E-cigarette users are heating and inhaling flavouring chemicals that were never tested for inhalation safety," said Allen.