By now most of us will have heard of the gas incident at Beachy Head, with at least 233 people hospitalised.  Chlorine starts to irritate the eyes at 30 parts per million - I would guess at least 10 cubic kilometres of air would be affected, so that's 300 cubic metres of pure chlorine, not allowing anything for uneven dispersion, which would raise the initial release amount 'required'. Seems a lot, or maybe it was something more potent than chlorine (odd that there wasn't much mention of a greenish cloud, or a 'swimming pool' smell).

Dr Hillary J. Shaw
Director and Senior Research Consultant
Shaw Food Solutions
Newport
Shropshire
TF10 8QE
www.fooddeserts.org