...Back to diacetyl/similar; from here:
CDC - NIOSH Update - Diacetyl and 2,3-pentanedione: NIOSH Seeks Comment On Draft Criteria Document
For Diacetyl, a recommended exposure limit (REL) of 5 parts per billion (ppb) as an eight-hour, time-weighted average, (TWA) during a 40-hour work week. To further protect against effects of short-term exposures, NIOSH recommends in the draft document a short-term exposure limit (STEL) for diacetyl of 25 ppb for a 15-minute time period.
For 2,3-pentanedione [acetyl proprionyl], a recommended exposure limit (REL) of 9.3 parts per billion (ppb) as an eight-hour, time-weighted average, (TWA) during a 40-hour work week. The REL for 2,3-pentanedione in the draft document is based upon the lowest level at which the substance reliably can be detected using the existing analytical method, and should not be misconstrued to imply that 2,3-pentanedione is of lower toxicity than diacetyl. Further, NIOSH recommends a STEL of 31 ppb to limit peak exposures for 2,3-pentanedione on the same basis of analytic method limitation.
How much is 5 - 25 parts per billion? It's 0000005% - 0000025%
We have been almost entirely unable to find the percentage of the diacetyl/like chemicals contained in typical flavorings (GRRRRR) - but we did find a few from Baker Flavors in Russia
http://translate.google.com/translate?hl=en&sl=ru&tl=en&u=http%3A%2F%2Fbaker-flavors.blogspot.com%2F2010%2F03%2Fe-cigarettes.html (which is Vaping Zone's Gourmet Line). The Tiramisu contains the most at .6% diacetyl of the ones they tested back in 2010...and it's commonly mixed at 2%. Let's make 100ml ::

praying our math is correct::: ...
100ml @ 2% flavoring = 2ml of Tiramisu flavoring in 100ml of liquid. 2 mls of Tiramisu flavoring contains .6% diacetyl (.6% x 2ml = .012mls of diacetyl)....so there's .012 ml of diacetyl in that 100mls of juice.
What's the percentage of diacetyl in that 100mls of juice? .00012%
Using this "ppb calculator"
Parts Per Billion Conversion Chart (Fractions and Percent Converter, Percentages And Parts), we find one must enter 1200 ppb to equal .00012%
So, vaping that particular flavoring at 2% means we'd be vaping 1200 parts per billion, when the 'safe zone' for short term exposure/inhalation is 25 parts per billion.
Yes, we DO have reason to avoid these chemicals. Even if they're 50 or 75% less than the example, we'd still be in the 300-600 ppb zone. Of course, the raspberry flavor starts at .00009% diacetyl, and that works out to WAY less than 1ppb (around .012 ppb, if I calculated quickly correctly).
'Always glad to see posters concerned about the health aspects of the chemicals we're inhaling!