In order to recover the well-known interference pattern in the double slit experiment with massive electrons, one has to perform it in a vacuo. This is because, as far as I know, the interaction with the environment (e.g. air particles) performs a measurement on the quantum states of the propagating electrons and the interference pattern gets washed out.
Now, from a physical point of view: as an observer, I have no idea what interactions the electron wavefunction is having the various air particles. So how can this result in an observable effect such as having the interference pattern changed?