"Exposure Rate" expresses the rate of charge production per unit mass of air and is commonly expressed in roentgens per hour (R/h) or milliroentgens per hour (mR/h).
Applicable units are those of quantitykind:ExposureRate
http://hps.org/publicinformation/ate/faqs/gammaandexposure.html
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD050
"Exposure Rate" expresses the rate of charge production per unit mass of air and is commonly expressed in roentgens per hour (R/h) or milliroentgens per hour (mR/h).
$\dot{X} = \frac{dX}{dt}$, where $X$ is the increment of exposure during time interval with duration $t$.
http://www.iso.org/iso/catalogue_detail?csnumber=31895