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We use Monte Carlo simulations to study the influence of gas adsorption on the conductance of nanocrystalline metal oxide films. The films are modeled by a network of intergranular contacts with conductances that depend on the amount of adsorbed gas molecules, and take into account a broad distribution of grain sizes and the possibility that ultrasmall nanograins can be insulating. Using percolation theory, we show that below a critical gas concentration (detection limit), the film can be insulating due to the absence of a percolating cluster of conducting grains. Above this detection limit, the conductance of the film increases rapidly. The detection limit can be tuned by the grain size and the mean coordination number in the film.