The leak was the fourth incident at the site in 18 months, Mr Barnes told the House of Commons during an adjournment debate. Chlorine escaped into the atmosphere after a valve was left open during a recommissioning operation.
The gas affected six workers, but Rhodia delayed setting the site's emergency plan in motion - indeed, it was ambulance staff who alerted other emergency services when called to the incident.
A warning siren was not sounded until at least 35 minutes after the chlorine leak ended, said Mr Barnes, when people in the locality were already suffering ill effects. "Many people suffered much discomfort, reporting runny eyes, hair that became like straw, and reddened complexions." Some 200 people took shelter in a supermarket, and others hid in schools.
Confusion was sown when people working in a major building in the town centre called Rhodia about the bleach-type smell in their offices. Mr Barnes said that the company responded "by suggesting the windows be opened to let the chlorine out - even though the procedures laid down require windows to be locked."
Residents near the site have also been advised to stay indoors until an all-clear siren sounds, but this part of the procedure was muddled as well.
According to Mr Barnes, the main warning siren was switched off after 45 minutes "to preserve the batteries", and the all-clear siren was not sounded for more than three hours.
An inquiry into the incident is being conducted by the Health and Safety Executive and Environment Agency, the joint competent authority under the COMAH regulations on major accident hazards.