of damage. No injury to plants occurs below exposure of 100 ppm for 1 week. Ethylene injury symptoms develop in plants only in exposure to high concentrations and take several days to develop. After exposure to the gas is stopped, level of recovery is variable in different species. Generally, younger plant parts recover but older parts do not. Much ‘acute’ damage to plants is caused on the fringes of polluted area or by a steady leakage of gas in low concentration. Impact of HCl pollution decreases with increase in humidity, deficiency of Mg and excess of Ca.