Reported field observations, laboratory studies and transplant experiments confirm that lichens are affected by pollutants. The ubiquitous nature of lichens and their susceptibility to the environment suggest that their response to variations in atmospheric composition might provide a useful means for monitoring environmental quality. Through such monitoring, the detection of abnormal levels of defined pollutants in the atmosphere would be an indicator of the need for remedial action before dramatic evidence of environmental damage becomes apparent. An additional advantage would be that previously unrecognized pollution sources affecting lichens and the wider environment might be detected. As lichens have important ecological and economic roles in our society, recognition of lichen communities at risk would be an associated benefit. The stable isotope abundance of carbon in the lichen Cladia aggregata has been investigated in a study relating these values with known levels of selected pollutants in the region within a 200-km radius of Sydney, Australia. A total of 129 samples of the sterile and fertile forms of the lichen was obtained from 36 different sites. There was an 8‰ range in the δ¹³C values of these samples of whole lichen. Small effects on δ¹³C values in the lichen, of less than ±1‰, were found to be associated with distance from pollution source, slope of the substrate site, humidity, altitude and number of rain days at the sampling site. The effects of the natural parameters tend to cancel out. A statistically significant negative correlation between δ¹³C values in the whole lichen and atmospheric sulphur dioxide concentrations was found. This is consistent with the known destructive effect of SO₂ on lichens. δ¹³C values in lichens in wet environments, in the presence of high SO₂ concentrations, were more negative than in samples from drier sites. The direction of this relationship is contrary to that observed in laboratory studies by other workers. An explanation of this effect may relate to an increase in carboxylation resistance resulting from damage to chlorophyll and cell structure by SO₂, which over-rides an inverse effect of CO₂ resistance that increases with wetting of the lichens. The predictive reliability of the graphical tool formulated from these data requires the accumulation of data for atmospheric SO₂ concentrations localized to sampling sites, followed by further testing.