How to make tmux child scope inherit environment variables from parent?

Question

I am using tmux version 1.8. How can I make sure that when creating a new window / panel, the newly created window inherits the environment variables that were set in the parent panel? Is it possible to do this? I found this solution, which was posted on the stack over the stream, but this did not work for me.

set-option -ga update-environment ' YOUR_VAR'