A gas detector comprises a carbon monoxide (CO) sensor for measuring an environmental CO concentration and providing a variable electrical output in response. A processor is responsive to samples of the electrical output for computing a carboxyhaemoglobin (COHb) level of an individual as if exposed to the measured CO concentration and providing an output signal based on the COHb level computed. A computed COHb level is obtained with each measurement sample. The computed level is associated with a current measurement sample being based on a relationship between a previously computed COHb level associated with previously measured samples and an equilibrium COHb level corresponding to the environmental CO concentration of the current measurement sample. The computed COHb level is an incrementally higher level than the previously computed COHb level when the equilibrium COHb level corresponding to the current sample is higher than the previously computed COHb level. The incrementally higher level is based on a rate of COHb level increase associated with the environmental CO concentration of a current measurement sample. Alternatively the computed COHb level is an incrementally lower level than the previously computed COHb level when the equilibrium COHb level corresponding to the current sample is lower than the previously computed COHb level. The incrementally lower level is based on an exponential decrease in COHb level from the previously computed COHb level towards the equilibrium COHb level corresponding to the current measurement sample. Alternatively the computed COHb level equals the previously computed COHb level when the equilibrium COHb level of the current measurement sample equals the previously computed COHb level. Also claimed is a method of monitoring CO concentrations in accordance with the gas detector described above. The step of computing a COHb level includes increasing or exponentially decreasing a previously computed COHb level associated with previously measured samples when the equilibrium COHb level corresponding to the current sample is higher than or less than the previously computed COHb level, respectively. The increasing is based on a rate of COHb level increase associated with the environmental CO concentration of a current measurement sample. The exponential decreasing is based on an exponential decrease in COHb level from the previously computed COHb level towards the equilibrium COHb level corresponding to the current measurement sample. The previously computed COHb level is maintained when the equilibrium COHb level of the current sample equals the previously computed COHb level.    USE -   for continuously monitoring levels of CO emitted from gas leaking pipes in gas fire, cooker etc.    ADVANTAGE -   More accurately correlates sensing of toxic carbon monoxide gas in closed environment with equilibrium levels existing in individual's blood stream and adjusting alarm signals accordingly. Minimises or eliminates false alarms.