The compound ozone, a known respiratory irritant, exists in high concentrations at flight altitudes, making the “fresh air” sucked in by air conditioners at those heights, well, not so fresh. In fact ozone exposure may be responsible for many of the short-term discomforts we associate with air travel. What’s more, ozone can react with other compounds in the air -- even the oils of our skin -- to produce other toxic compounds, like aldehydes and ketones. Some planes have catalytic converters, like the ones in cars, which use transition metals to turn ozone into breathable oxygen. But not every plane has one!