"Dirty" is a pharmacological term, if, admittedly, an "informal" one,
as Wikipedia phrases it:Amusingly, dextromethorphan as you can see is actually listed as a prime example of a "dirty drug".
Why would you think this? Dissociation is not a medically desired state, thus the majority of dissociatives that are also in medical usage have not been studied for their psychiatric effects.
This information is readily available everywhere. The
Erowid DXM dosage page puts a "Risk of Death" dose between 2,500 - 20,000mg. Note the extremely wide range and how close this is to the "Heavy" recreational dose range which tops out at 1500mg. I won't bother to link you to the ketamine dosage page but recreational dosages of ketamine are known to be around 250-500mg at the higher end, which is still well below the usual anaesthetic dose used in medical settings, and the lethal dose of ketamine is not even known, it's widely known to be a very safe substance - when speaking in terms of acute toxicity from a single dose.