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I've been trying to create a Earth-like planet but I'm not sure if my planet's atmosphere is breathable. (It's mostly the amount of helium I'm worried about).
I've tried to make it Earth-like while making it alien with respect to Earth's atmosphere. Here are the atmosphere's numbers: atmospheric pressure: 1.22 atmospheres; atmosphere composition: 66% nitrogen, 19.42% oxygen, 14.21% helium, 1.25% xenon, and 0.12% other gases.
Q: Would this type of atmosphere be suitable for human life?
Given that you've increased the atmospheric pressure, the slight drop in O2 won't be a problem for breathing but your problem in this mix isn't the helium; it's the xenon.
Helium gets a bad rap because some people do stupid things with it; for fun. Yes, breathing helium in large quantities makes your vocal chords vibrate faster because it's less dense than normal atmosphere, so you sound like a chipmunk. The problem is that you need O2 in the air to survive. Breathing in massive amounts of pure helium suffocates you because you don't get any O2 by way of breathing it in. That said, it's not unsafe to breathe as part of the atmosphere and deep sea divers actually do exactly that; Heliox is a gas mixture of both O2 and He2 (instead of O2 and N2) because at pressure, the amount of nitrogen you breathe can make it toxic. Diving to even a relatively shallow depth of 30m using normal air brings on something called Nitrogen Narcosis and it affects your judgement et al like alcohol does.
Xenon, on the other hand, is also safe to breathe in its pure form but it has a singular disadvantage; it can be used to form many different compounds with O2 and many of those compounds are toxic to breathe and very exothermic, read as explosive.
In response to comments, it should be noted that these compounds are VERY hard to form and involve a significant amount of fluorine, more than would be allowed within trace gases. The paragraphs that refer to these reactions have been deleted, however the remainder of the answer is still valid.
The Xenon does however have a narcotic effect in its pure form, just like Nitrogen does in larger quantities than normal atmospheric pressure afford. Xenon is about 25x times more narcotic than nitrogen, so in the quantities listed above your colonists may not be able to function very well, if at all. It would be like living in a mist of anesthetic, and in point of fact xenon isn't used for anesthetic purposes in medicine now only because of its high cost.
Add to that you're dealing with gasses with very different densities. Xenon is heavier than normal air, helium is lighter. If the gasses start separating as a result (very likely) you could actually have a suffocating layer at different altitudes, and mine shafts in particular could be very dangerous as over time they'll fill with much more xenon than normal gasses.
Also, to have that much helium in the atmosphere in a stable form, you've probably got a LOT of exposed decaying uranium. So, while you have helium in abundance, you also have radiation.
All things considered, I'm not convinced this atmospheric model is stable, or safe. The helium isn't the real problem, but the way it's created might be. The xenon has the opposite problem insofar as it's not dangerous in a pure form, but what it becomes might be.
My recommendation? Have your colonists wear rad-safe spacesuits until they've done a more complete study of the atmosphere. Better safe than sorry.
That seems perfectly breathable to my knowledge: helium, xenon, and other noble gasses don't tend to interact with anything (hence "noble"), and so the only problems they cause have to do with them displacing other things. For instance, a person can breathe 100% helium without it causing any health problems—the issue is the 0% oxygen.
However, your planet will need stronger gravity than Earth in order to hold helium in the atmosphere. Earth's gravity isn't strong enough to keep hydrogen and helium around, so they'll dissipate off into space over time. (We get helium nowadays from nuclear fission happening deep underground, and hydrogen from breaking apart water, rather than extracting them from the atmosphere.)
You'll also want CO₂ for the plants to use, but that would fall under "other" (and you don't need very much). And you probably want some water vapor too, but that doesn't directly impact its breathability.
This atmosphere is lethal to humans.
It has too little oxygen partial pressure 11 kPa vs 21 kPa (Normal) vs 16 kPa (Minimal safe) vs 13.3 kPa (hypoxia).
That is because you have too much helium partial pressure (66 kPa). And that is because helium has too little molecular weight (4) compared to oxygen (32) which increase a lot the helium value.
You can solve this by: increasing oxygen percentage (a lot), reducing helium percentage (also a lot), increasing atmosphere pressure (a lot again) or changing helium to a heavier sustance (great!).
May I suggest replacing Helium wirh Argon?. That would raise oxygen from 11 kPa to 22 kPa making it literally perfect. The best part is that 13 kPa of argon won't produce negative effects and the 86 kPa of nitrogen will be close to nitrogen narcosis but won't produce it!.
Finally, Xenon isn't lethal in that amount, but remember that it's heavy. Without wind it can accidentally settle on a plain or valley and asphyxiate everyone.
First I will calculate your atmosphere partial pressures:
You said $1.22\text{ atm}$ but I don't like atmospheres measure, so I'll convert them to kilopascals: $$1.22\text{ atm} \times 101.325 \text{ kPa} = 122.3965 \text{ kPa}$$
Also, since I don't know what is that $0.12\%$ "other elements" I'll assume it's more nitrogen.
$$\begin{array}{|cr|r|cc|c|} \hline \text{Chemical} & \text{Percent} & \text{gr/mol} & \text{Mols} & \text{Fractal Mol} & \text{Partial Pressure (kPa)} \\ \hline \text{N}_2 & 66.12\% & 28.0134 & 2.3602 & 0.3616 & 44.2617\\ \text{O}_2 & 19.42\% & 31.9988 & 0.6068 & 0.0929 & 11.3809 \\ \text{He} & 14.24\% & 4.0026 & 3.5501 & 0.5439 & 66.5753 \\ \text{Xe} & 1.25\% & 131.293 & 0.0095 & 0.0014 & 0.1785 \\ \hline \text{Total} & 100\% & - & 6.5269 & 1 & 122.3965\text{ kPa} \\ \hline \end{array}$$
Now, lets start to analyze!:
Now, that you have read all my long explanation please read my TL;DR part, there is explaining of what is going on.
If you follow my suggestion, this will be your new atmosphere:
$$\begin{array}{|cr|r|cc|c|} \hline \text{Chemical} & \text{Percent} & \text{gr/mol} & \text{Mols} & \text{Fractal Mol} & \text{Partial Pressure (kPa)} \\ \hline \text{N}_2 & 66.12\% & 28.0134 & 2.3602 & 0.7082 & 86.6911\\ \text{O}_2 & 19.42\% & 31.9988 & 0.6068 & 0.1821 & 22.2906\\ \text{Ar} & 14.24\% & 39.948 & 0.3557 & 0.1067 & 13.064 \\ \text{Xe} & 1.25\% & 131.293 & 0.0095 & 0.0028 & 0.3496 \\ \hline \text{Total} & 100\% & - & 3.3324 & 1 & 122.3965\text{ kPa} \\ \hline \end{array}$$
Let's take a step-by-step approach. Thankfully for you, there is a lot of information on the various effects of different gas combinations on the human body, most of it somehow relating back to SCUBA diving.
This is around 17 PSI.
This doesn't pose a significant problem. Earth's atmosphere is already ~70% nitrogen, so this isn't too far from the norm. There does exist a condition called "nitrogen narcosis" (see xenon), but at below 2 atmospheres, there are hardly any noticeable effects.
Long exposure to oxygen at increased partial pressure results in a lot of bad things happening to your body.
Normally, more extreme effects are experienced in very high oxygen concentrations and pressure several times normal. However, your colonists will always be exposed to more oxygen than they need. The effects of breathing such a mixture for your entire life is hard to research, but some of the effects of oxygen toxicity (retinal damage, hormonal abnormalities, etc.) can be anticipated.
Helium is very light. Unless your planet has much, much higher gravity, all your helium will drift off into space. Even if the helium stuck around, it would be most concentrated near the top of the atmosphere, and doesn't have many interesting effects on humans anyhow.
However, the fact that your atmosphere is not evenly mixed would mean that traveling to high altitudes would be dangerous, because after a certain point the air would not be able to sustain human life.
This part causes trouble. Xenon is significantly heavier than Earth air, and will sink to the bottom of the atmosphere. Xenon is a noble gas and is not reactive, but will act like a narcotic at higher concentration (and especially at the elevated pressure). To quote Wikipedia:
The noble gases argon, krypton, and xenon are more narcotic than nitrogen at a given pressure, and xenon has so much anesthetic activity that it is a usable anesthetic at 80% concentration and normal atmospheric pressure.
Although 1% xenon may seem reasonable, remember that your atmosphere is noticeably higher pressure than Earth, and that your colonists will always be breathing this air. Without some other source of air, it would be pretty hard for your colonists to function well.
Overall, xenon has plenty of interesting effects on the human body. There is plenty more information on its dedicated page on Wikipedia.
