It's molar concentration, the study doesn't really mention it directly of course. 1mM is probably high since they're looking at what concentration of tobacco causes DNA damage in e.g., a cavity, but the study doesn't think it's unreasonable. Other studies in animals used up to 4mM.

The full study can be found as the first article in this pdf: http://tobacco.cleartheair.org.hk/wp-content/uploads/2018/01...