'On the tongue, bitter substances trigger taste cells to stimulate neurons, which then evoke a response -- the perception of a bitter taste. In contrast, the airway cells appear to use a different mechanism that does not require nerves,' Shah said.

'In the airways, bitter substances both activate the receptors and elicit a response -- the increased beating of the cilia -- designed to eliminate the offending material.'

Shah's co-author Yehuda Ben-Shahar said: 'These findings suggest that we have evolved sophisticated mechanisms to guard ourselves from harmful environmental stimuli.

'Our work also suggests that losing cilia in the lungs, due to smoking or disease, would result in a reduced general ability to detect harmful inhaled chemicals, increasing the likelihood of further damaging an injured lung.'

These findings were published online in Science Express.