For example, the command

execute @a[y=75,r=10] ~ ~ ~ particle dragonbreath ~ ~ ~ 1 1 1 0 100 @s

within a command block will begin emitting the dragonbreath particle effect when players are within a 10 block radius of y=75. That exact command in an mcfunction will not work, however.

My functions are set up properly; using a different target selector argument will trigger the mcfunction when true.