Different gases will have VERY different minimum concentrations x times of exposure to represent a human health hazard. Gas monitors will claim resolutions as high as 0.1 ppm to 0.1 % vol. Accuracy, however, is affected by conditions at which calibration takes place in comparison to actual measurement situation. Also, when there are several gases present there is a cross sensitivity issue reflecting the loos of accuracy due to the presence of different gases that can react to the analyzer. A list of gas detectors is also here http://www.intlsensor.com/gaslist.html