Even though some lichens are extremely tough and grow in very inhospitable habitats, they are also notoriously sensitive to air pollutants, primarily sulfur dioxide and heavy metals. Lichen deserts, a phenomenon where lichens disappear from cities, were described over a hundred years ago and determined to be caused by sulfur pollution. Lichens are especially sensitive to air pollutants because they have no outer impermeable layer of tissue to exclude gases and particles that impair their metabolism. Consequently, accumulation of pollutants is greater than it is in the foliage of vascular plants, which have impermeable cuticles. Lichens accumulate unusually large amounts of deposits, including heavy metals, which eventually reach toxic concentrations. Lichens are therefore excellent bioindicators and biomonitors.

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