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Those chemicals include diacetyl, a diacetyl-alternative called acetyl propionyl, acetoin, and diketones.

In total, 51 types of flavored e-cigarettes and liquids sold by leading brands were tested for diacetyl, acetoin, and 2,3-pentanedione.

The Harvard researchers looked for the presence of diacetyl in 51 different e-cigarette flavors, as well as two other flavoring chemicals, acetoin, and 2,3-pentanedione.

One such flavoring chemical, acetoin, was detected in 46 of the flavors while another, called 2,3-pentanedione, was found in 23 of the flavors, the findings showed.

They tested the e-cigarettes for the presence of three chemicals considered “high priority” when it comes to respiratory risk: diacetyl, acetoin and 2,3-pentanedione.