Apple has released a firmware update that is recommended for all MacBook Air (mid 2012) models. The update webpage reads:
Apple has discovered that a small percentage of flash storage drives in these MacBook Air models have an issue that may result in data loss. This update tests your drive and, in the majority of cases, installs new firmware to resolve the issue. If your drive cannot be updated, Apple will replace it, free of charge
The update will not show up in your Software Update list. To download the update get it directly from Apple’s website here.
UPDATE: according to Apple documentation the update should now show up in Software Update. See the link at the bottom of this post for more info.
I recommend (if you haven’t already) backing up all your data before applying this update. The update may tell you it’s not needed on your system, install or fail. In the first two cases you should be good to go but if the update fails, have the solid state drive replaced sooner rather than later.
More info also here