# Is a magnetic field affected by the presence of air?

Does air affect a magnetic field's ability to attract ferromagnets? If a magnetic field and a ferromagnet were placed in a vacuum would there be a better performance?

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The magnetic permeability of air is negligibly different than vacuum, so probably not. –  DumpsterDoofus Feb 14 at 4:24
@DumpsterDoofus please explain this a bit more. –  Key Feb 14 at 7:50

Oxygen in air is a triplet diradical, ↑O-O↑, a a paramagnet. Its slightly greater magnetic permeability than vacuum would slightly concentrate the field. Compare magnetic susceptibilties,

Nitrogen (1 atm) $-0.5\times 10^{-8}$
Oxygen (1 atm) $209.0\times 10^{-8}$

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In the first case, you have a field emanating from a magnet, traveling through air ($\mu_r=1.000000349)$, and entering a ferrous material ($\mu_r\approx4000$).
In the second case, you have a field emanating from a magnet, traveling through vacuum ($\mu_r=1)$, and entering a ferrous material ($\mu_r\approx4000$).