I recently met the people who run this website:
To give you a quick version: apparently in commercial planes, the air you breathe in the cabin in brought into the plane through the engines, where it is exposed to all of the toxins and nasty shit that is present in airline fuel and oil. This means the quality of the air in the cabin is basically a horrible mess of carcinogens and other stuff you don't want to breathe in. While it's bad for everyone, some people are especially susceptible, including infants.
The guy I met was a pilot for 25 years and said he can directly attribute poor health to this, along with dozens of other pilots and crew members around the world. He knows of several deaths of otherwise healthy people due to it. But, he said that because the airline industry make so much money, and is so tied to governments and research institutions etc, there has never been a full expose of it, with no newspaper willing to touch it. He reckons that there's finally going to be some movement this year which will expose it as one of the biggest public health scandals in decades, so watch this space.
What do you reckon? My instinct is always to think it's a load of crank, but then again, it's all so plausible. He offered to send me some breathing masks anyway, so I'm sorted.