  In the Milky Way galaxy, positrons, which are responsible for the diffuse
$511\,\mathrm{keV}$ gamma ray emission observed by space-based gamma ray
observatories, are thought to annihilate predominantly through charge exchange
interactions with neutral hydrogen. These charge exchange interactions can only
take place if positrons have energies greater than $6.8\,\mathrm{eV}$, the
minimum energy required to liberate the electron bound to the hydrogen atom and
then form positronium, a short-lived bound state composed of a
positron-electron pair. Here we demonstrate the importance of positron
interactions with neutral alkali metals in the warm interstellar medium (ISM).
Positrons may undergo charge exchange with these atoms at any energy. In
particular, we show that including positron interactions with sodium at solar
abundance in the warm ISM can significantly reduce the annihilation timescale
of positrons with energies below $6.8\,\mathrm{eV}$ by at least an order of
magnitude. We show that including these interactions in our understanding of
positron annihilation in the Milky Way rules out the idea that the number of
positrons in the Galactic ISM could be maintained in steady state by injection
events occurring at a typical periodicity $>\mathrm{Myr}$.
