How concerned should I be about these slightly swollen batteries?

Hi all,

I noticed a slight wobble in my MacBook when I set it down on my desk. I opened it up to check on the batteries and noticed a slight pressure on the backplate while I was unscrewing it, all but confirming my fears. And sure enough, as you can see in the picture my batteries are swollen; but only slightly. It’s slight enough that it’s hard to capture on camera without a light background, but if you look at the rubber pads you can see they are not level.

My question is: how concerned should I be about this level of swelling?

It’s not nearly as severe as the Google Image results for swollen batteries, but they are swollen enough to be noticeable and put pressure on the backplate. All batteries have a very slight bulge to them, the one in the picture is the most severe. I’ve powered the laptop off and unplugged it from the charger.

• Am I being paranoid or am I right to be concerned? Is this an “acceptable level” of swelling, or should I immediately stop using the device?
• Are the batteries in danger of catching fire at this stage? If so, what steps should I undertake to keep it safe?
• Is it still safe to power it on briefly to backup any files that aren’t already backed up or should I leave it turned off?

Thanks for any help you can give me answering these questions.