I'm looking to build a horror type sci-fi game based on the Alien franchise. My question comes in trying to differentiate my alien from the Xenomorphs. I want hurting it with a melee weapon to be inherently dangerous, but not use acid as blood. So my first idea was to go the other way on the pH scale, but I don't have enough of an understanding of chemistry. So, my questions are:

How basic would a substance have to be to cause immediate damage, and what would the damage look like (basic blood instead of acid blood)? I'm not looking for something that burns through everything on contact, just something to give pause to someone whose options are mostly stabbing-related.


What would the major difference be between having something spill acid vs spill a base when it bleeds?

Also let me know if these questions are too different despite being related, and I'll split this up into 2 questions.

Thanks for your time!

  • $\begingroup$ Welcome to the site, Glenn! Since blood is a natural base (at least for humans), I assume you want to dial the effect up to 11? $\endgroup$
    – Frostfyre
    Jan 22, 2020 at 21:09
  • $\begingroup$ @Frostfyre Yeah, I'm looking for something basic enough to injure a human on contact, if that exists. That something not existing is also a valid answer $\endgroup$ Jan 22, 2020 at 21:10
  • $\begingroup$ A base strong enough to cause severe injury on contact is definitely possible, lye for instance. $\endgroup$
    – Rekamanon
    Jan 22, 2020 at 21:18
  • 1
    $\begingroup$ @Frostfyre You'd have to dial it past 11! An ammonia solution has a pH of about 11, which can irritate the skin, but its effects won't be particularly cinematic. In this particular case, you might need to dial it up to 14. $\endgroup$ Jan 22, 2020 at 21:36
  • 1
    $\begingroup$ @NuclearWang But my dial only goes to 11... $\endgroup$
    – Frostfyre
    Jan 22, 2020 at 22:43

2 Answers 2


Suggesting a Reframe...

Not and Be Science Based

The "molecular acid" (meaningless sci-babble) that ate through bulkheads couldn't reasonably have run through the veins of the alien in Alien, but Alien wasn't really meant to be hard sci-fi. That no one could hear you scream was a nice touch, but there's lots about the series that's definitely not science-based.

That said, whether caustic or acidic, things too far off neutral have similar effects in contact with many materials. Oven or drain cleaner is typically a strong base, and either of those will injure a human on contact. Lye can dissolve bone. The major difference would likely be metals. Typically, a base is a hydroxide group bound to a metal, so metal can safely be used to store most bases, while acids react with metal to form a metal salt and hydrogen.

That said, even the most terrifying superacid and the most awful superbase won't do what the alien's blood in Aliens does. They might do worse things, mind you (strong bases diluted in water are powerfully exothermic, which might be exciting, and strong bases can do truly awful things to organic compounds).

So I'd remove "science based" if you want blood like the Xenomorph's, but if you're content with "horrifying eventual reactions", a pure aqueous solution of hydroxyl ions would not make anyone splashed with it very happy. It wouldn't work as blood, as far as we're familiar with the stuff, but it would definitely ruin some space traveler's day if they're dumb enough to open their helmets. Again.

Edit: Even with the question edit, concentrated sodium hydroxide will do the trick. Effective pH > 14. The most basic it can be is ~15.3 (saturated solution of NaOH in water), but anywhere in that ballpark will burn skin on contact. A good spacesuit would mean that the stabber doesn't care, though.


For a science-based question, I'll tackle it slightly differently and re-frame it. It doesn't make a ton of sense for there to be living organisms with acid for blood like the xenomorphs.

So, I'd take a different reading of your question: you want your aliens to be dangerous if causing open wounds because of what the (human) stabber will be exposed to. So, not necessarily blood and not necessarily alkaline content.

They have a substance in their body their that is toxic to humans

Now, I'd discuss a number of possibilities here. I'm going to pick the ones that seem the coolest to me and further explore them, but you can always go in a different direction from something I've mentioned but not expanded on.

What is the substance?

This could actually still be blood. But it might not be, depending on physiology. They could have, say, a subdermal layer of the substance that helps them somehow. It might be to keep them warm, it might be specifically to protect from injuries. Maybe it's even a defence mechanism against predators. It might also be a property of their blood, not necessarily a separate substance. You can tailor as appropriate.

Why is it dangerous?

The simplest answer is that maybe it's not even supposed to be but it's dangerous to humans. But we can delve a little deeper.

There are species of animals in the real world that have evolved defences like these. In their case, it's usually against being eaten. There are the poisonous animals like poison dart frogs. The rationale is that if they are eaten, the predator will suffer greatly or die. This doesn't help the individual animals, but over time there won't be many predators left that will prey on the poisonous animals. In addition, poisonous animals tend to be very brightly coloured as a "don't eat me" mechanism. This all means that over time, the poisonous species will be protected from predators.

Just because this is cool, I'll add another example - the skink is a lizard with green blood and most internals. It's apparently green because it contains a large concentration of a chemical called biliverdin which is toxic. From the article:

If humans have even a tiny amount of biliverdin or bilirubin in their blood, we say that they are jaundiced (their skin takes on a yellowish tone).


if excess bilirubin or biliverdin in the bloodstream goes untreated, it can be deadly.

“It’s surprising because at these concentrations of bile pigments in the blood, [the skinks] should be completely jaundiced, if not dead,” Austin said.

The researcher who studies skinks thinks that is an evolution to protect from parasites, rather than predators.

With all that said, it's not at all unusual to have a substance that is dangerous to others. Or maybe just to humans, but this would suggest it's rather random...unless like the xenomorph, the alien adapts to given species.

How did it evolve?

A defensive mechanism seems a very likely candidate. It needs to be work through skin contact not necessarily through being consumed. This makes it eligible for a "coating" of the substance on the skin so just touching might trigger poisoning the attacker but any open wounds might still spread the defensive toxin. Perhaps instead it's being shot out by glands on the body that gets triggered the creature is attacked (I assume automatic, rather than at will).

Still, if you insist on open wounds being that thing that spreads the toxic substance, this is doable and also has interesting implications - a very strong defensive mechanism that serves as an extreme deterrent to open wounds, suggests there is dangerous fauna (or maybe even flora?) in the aliens' natural environment that causes puncture wounds. So, species which use claws, horns, or even teeth, as well as other spiky or cutting parts to cause open wounds. Such methods of causing injuries will be countered very well by having the attacker suffer for spilling blood.

This can give you an idea of what other species native to the main alien's world there would be. Or maybe shape the ability based on what you've already figured the alien fauna is.

At any rat, since toxic substances are likely to be slower acting than directly burning enemies with heavily acidic or alkaline blood, then this also has interesting implication - the alien with this defensive mechanism: it can survive well even when injured, for the defensive mechanism expects the attacker to first cause a wound and some time later they'd suffer for it. The target of the attack has to survive for the time the poison takes effect.

  • One option is that a wound not only delivers a toxin to the attacker but also causes the target to get an adrenal (or equivalent?) burst, so they can either escape more easily or fight and defend themselves better.
  • Another option is that the alien has evolved to be very sturdy. Likely there are redundant organs and very thick external layer. In this case, the poison could be a subdermal layer that serves as extra protection. For example, it could be useful for shock absorption thus lessening the effects of any blows that don't pierce outer layer or don't pierce it enough. If an attack opens a "wound" (it's likely for the outer layer to be more defensive, so opening it is not going to equate to damage to the body) then the toxic substance leaks (or shoots) out as a retaliation. The alien can still fight the attacker relying on outliving them.

In conclusion

I picture a sturdy alien that is probably generally slow to run away from natural predators in its environment but with very good endurance, resistance to pain, and survivability. If it gets into a fight, it relies more on outlasting opponents than just beating them through sheer strength or speed. It has a subdermal layer of toxic substance that can help with shock absorption and if the epidermis is punctured, the shock absorbing substance coats the attacker seeping into their system and poisoning them. This serves as a way to shorten the fight - causing open wounds is bad, no matter how good your outer layer is - if an attacker can open it, then might be able to attack through that opening. So, the poison is a way to deal with this more directly and reduce the window the attacker has to cause serious damage. Eventually, they would be incapacitated by the effects of this defensive poison and the poisonous creature is expected to be sturdy enough to survive that long.

Overall, perhaps this could be loosely like a hippo or rhino - quite well defended and not a pushover. I'd expect them to be able to survive even a decent cut or a piercing wound. But they are also poisonous.

When facing humans, this works the same way - if you were to cut them with a knife or other similar weapons, that's not much different from what other animals will attack with. A good suit will protect the human from the toxic substance but they still have to deal with actually killing the alien creature which should take quite some effort. Attacks that don't cause open wounds are not that effective, so if shooting them, normal bullets might be at least partly mitigated if the outer layer of the creatures is hard enough to stop them, then the subdermal layer will additionally absorb some of the kinetic force. Using armour piercing ammo will likely work but they generally cause far less damage to tissue because they tend to go straight through. Also, shooting the alien full of holes will turn them into a sprinkler for poison, you better stand back and only examine the body once properly protected.


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