Working out exactly what the backpack number means in Once Human might take you a little while. It's kind of unnerving the first time your rucksack—or Cradle as the game calls it—starts beeping and displaying a steadily climbing number. Usually, a counter like this would represent your proximity to something important, perhaps even a hidden treasure.

In Once Human, however, it's more like when the radiation sensor on your Pip-Boy in a Fallout game starts going off: big numbers and lots of sound means stop and turn around. If you're just starting out for the first time, you might want some help changing worlds or some Once Human codes for free stuff, but otherwise, here's that number on your backpack explained. 

So what does the backpack number in Once Human mean?

Essentially, when your backpack starts counting upwards it indicates Stardust pollution in the nearby area—that stuff that's turned everyone into freaky monsters. When you enter a polluted area you'll usually also get a message saying that “The Stardust pollution is getting worse”. 

This is important because Stardust pollution reduces your sanity, indicated by the little dial with the head symbol at the bottom of the screen. This stat is pretty vital since reduced sanity means reduced health. The worse the pollution, the greater the health reduction you'll get when exploring polluted areas. While the number will tick up as you get further into the polluted area before capping out, what you really need to worry about is the colour on your backpack.

There are three colour levels of Stardust pollution severity. Here's what each means:

• Green: You’re well protected and your sanity and health reduction is minimal
• Amber: You could be better protected and your sanity and health are taking a bigger hit
• Red: Your sanity and health are massively reduced

If you want to see this in action, simply unequip your gear in a polluted area. Since gear provides the pollution resist stat, removing it will often take you down to amber or red level. So, what can you do to stave off pollution? Simply find and equip gear with decent pollution resist stats and make sure to keep it repaired. As you find more heavily polluted areas later in the game, you'll need even better gear to be able to tackle them.

I tried eating Sanity Gummies while in a polluted zone to get my health back, but it was kind of pointless. Until you're either protected by gear or leave the area, your sanity will just decrease by that fixed amount again, leading to that same health reduction. The most important thing is making sure you have adequate pollution resist before venturing too deep in the first place.