  In this work, we study the decay of mass for solutions to the heat equation
in exterior domains, i.e., domains which are the complement of a compact set in
$\mathbb{R}^N$. Different homogeneous boundary conditions are considered,
including Dirichlet, Robin, and Neumann conditions. We determine the exact
amount of mass loss and identify criteria for complete mass decay, in which the
dimension of the space plays a key role. Furthermore, the paper provides
explicit mass decay rates.
