Worksheet change  is a good candidate, and aesthetically more "nice"
than the idea of an asynchronous routine fire-off.

However (i.e. no free lunch), the worksheet change event is a bit scatter-gun,
in that it fires with a change in any cell, whether you're interested in that
one or not. So you've then got to work out whether you should do the work on
the cell. I certainly use this approach in some instances, and ignoring
unwanted cells  isn't particularly cumbersome.