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It is my understanding that when ionic species (such as $\ce{He^2+}$, or alpha particles) enter the human body, they tend to react with biomolecules, causing damage by (for example) mutating DNA.

If such ions can cause severe damage to the human body, why are other ions like $\ce{Na+}$ or $\ce{Cl-}$ safe to consume and not prone to causing similar mutagenic effects?

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    $\begingroup$ $\ce{He^2+}$ whats his electrons back very badly. $\ce{Na+}$, not so much. There are plenty of ionic species in the body. To say they are harmful is a really unjustified generalization, much like saying that all *.exe files are viruses. $\endgroup$ – Ivan Neretin Sep 13 '18 at 11:48
  • $\begingroup$ Hmm.. That's a good analogy :) But can you please elaborate why these other ions are not causing any harm to dna molecules or any other effects for that matter? $\endgroup$ – vivkv Sep 13 '18 at 12:23
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    $\begingroup$ Plus, an alpha particle typically has a lot of energy and loses it by bashing electrons and other nuclei around pretty violently. If you sent an MeV Na ion into you, it would do similar things. $\endgroup$ – Jon Custer Sep 13 '18 at 12:58
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    $\begingroup$ It's the difference between holding a bullet in your hand and getting shot by one. $\endgroup$ – Zhe Sep 13 '18 at 13:17
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    $\begingroup$ @VivekKumarPathak - No you have entirely the wrong notion. An alpha particle isn't just a $\ce{He^{2+}}$ ion, it is a $\ce{He^{2+}}$ ion with tremendous kinetic energy. In other words it is zipping along like a bullet. The alpha particle sheds its kinetic energy by knocking into electrons which creates all sorts of ions, and rips apart molecules in ways that won't happen via any normal chemical process in our bodies. $\endgroup$ – MaxW Sep 13 '18 at 16:49
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You are confused about the cause of mutagenesis in alpha particles.

The reason why an alpha particle is mutagenic is not that it is an ion, it is that it is an ion carrying a great deal of kinetic energy. Alpha particles are produced in energetic nuclear reactions and carry a great deal of energy away from the nucleus that emits them (a typical alpha particle has about 5MeV of energy, enough to break a typical chemical bond). It is the kinetic energy that matters not what the ion consists of. Heavier ions with that sort of energy would be just as bad for biological systems as are other forms of energetic radiation (beta particles are just electrons but will still do damage, gamma rays are just powerful photons but will dump enough energy into molecules to break DNA).

Non-energetic helium nuclei would pick up electrons extremely rapidly from the first molecules they encountered (probably from air even before reaching your skin) and would probably be as harmless to you as the helium filling a balloon).

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I am certainly no expert in this area but this is my understanding.

Let's consider an analogy of electrons. There are low energy electrons that move reasonably freely through a lump of iron for example. The electrons in it are not harmful, we can even eat this if it is small, in fact it is nutritious. We wouldn't normally call these electrons beta particles. Then there are high energy electrons released by a nuclear process. We call these beta particles, and they can smash through numerous atoms, causing many random chemical changes. This is a cause of radiation sickness and possible death. As you can see these have completely different characteristics because of the different amount of energy they have.

The situation is the same for ions. Common, low energy ions, trapped in a chemical substance, for example sodium ions in water, are very different from a high energy He2+, liberated by a nuclear processes, not bound up in a chemical substance.

Ultimately the important difference is physical rather than chemical.

There is one other aspect: ions like Cl- are only safe when they are in a substance that is neutral overall (for example Cl- with Na+). If you were to be exposed to pure Na+ it would be extremely harmful.

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