There are three bodily fluids that you can collect and arm against BTs in. They're getting more and more disgusting, so maybe I'd better leave the full explanation below. Just know that a toilet is 100% involved. The best part about these non-lethal tools? They're completely free, and you have an unlimited supply if you're willing to wait.
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How To Fight BT With Your Body Fluids | Survival guide
At some point in the story, Heartman will contact Sam to offer to collect his bodily fluids. Indeed, it is very early in history, and what he means is not very clear.
This is what he means: Depending on how you use the toilet in the private room, you can earn one of three different EX grenades that are effective at clearing BTs. You can select "Take a shower", "Use the toilet" or "Use the toilet".
There are three types of EX grenades. EX Grenades # 0, Ex Grenades # 1 and EX Grenades # 2 - and each is more powerful than the last to repel BTs. Taking a shower earns the number 0 from Grenades EX, while (naturally) the number 1 comes from 'Standing' and the number 2 from 'Sitting'.
Grenades are available in your private locker, regardless of the settlement you originally generated. Take a few and give them a try if you plan to enter areas infested with BT. It might make your life easier.
By sleeping in the private room, you will also be able to earn free blood bags. Blood bags are a must if you want to use Airtight Grenades without hurting yourself later.