Problems where impact broadened radio recombination lines appeared narrower than predicted first showed up \sim 40 years ago at frequencies below \sim 3 GHz .
But it was soon found that the observations could be explained by throwing out the uniform density models and replacing them with variable density ones .
However , this problem re-appeared recently when a mysterious line narrowing above quantum numbers of ( n, \Delta n ) = ( 202,8 ) was reported from sensitive observations of Orion and W51 near 6 GHz .
Here it is demonstrated that the narrowing is unlikely to be caused by the data processing technique and therefore must be source related .
It is further demonstrated that the observed line narrowing can be tied to one of the fundamental properties of radio recombination lines ; namely the fact that the spacing of adjacent n -transitions increases with frequency .
The line narrowing is observed to begin when the n -transition density , D _ { n } , exceeds \sim 11.6 transitions per GHz .
This may imply that it is somehow related either to a previously overlooked effect in the impact broadening process , or to some unknown parallel process , that is tied to the separation between adjacent n -transitions .
Based on these results it can be concluded , as has also been concluded in several theoretical investigations , that the observed line narrowing is not tied to a fixed range of either n or \Delta n .