Hi Mark,
Since you are mentioning the mapping mdb: Does your problem occur when you load regulatory data or when you have the rule sets calculate data? If you are talking about rule sets, do the following:
In the ExpRecordMap, find the corresponding entry and change it from non-phrase to phrased. In ExpPhraseMap, maintain an ew entry with the internal ID mg/m3 (standard notation) and the external ID mg/m3 (your notation).
Ralph