Chlorine levels at a leisure centre fell dangerously low on the day a sickness bug outbreak led to scores of people being violently ill, an investigation found.

Children and adults caught the vomiting bug after visiting Splashes in Rainham on Friday, October 28, last year. It is estimated about 120 people were unwell and 125 days of work or school were missed as a result of the illness.

As reported earlier this week, an investigation by Public Health England (PHE) found staff did not follow standard pool procedures.

As the report has not been made public by Medway Council or Public Health England, we have taken the decision to publish it. You can read the full report here.

On the day of the norovirus outbreak, the automatic chlorine system was turned off at noon to allow a coloured dye to be put in the water for a Halloween-themed pool party. In order for the pool dye to work, chlorine levels have to be below 1.0ppm (parts per million).

The council’s normal operating procedure requires levels to be kept between 1.5ppm and 2.5ppm, and should not fall below 0.5ppm.

Records show at 7am, chlorine levels were 2.09ppm but by 2pm this had fallen to 0.24ppm.

According to the PHE report, the manager turned the automatic dosing machine back on for at least one and a half hours and chlorine levels were monitored, but none of this was recorded.

The reason for this is still under investigation but the manager told PHE a computer failure had significantly disrupted routines.

No more chlorine recordings were made until 7am on Saturday, when levels were 0.0ppm.

PHE became aware of the outbreak when they were contacted on Monday, October 31, by Thames View Primary School and the media, including this paper.

The school, in the same road as the leisure centre, suspected 16 of their pupils fell ill after going to the pool.

Medway Council has disputed the figure of how many people were affected. They say they had 31 reports equating to about 60 individual cases, but a formal count on Facebook suggests it was around 120 cases. A questionnaire set up by PHE had 93 responses.

Recommendations by the PHE include:

• Additional training for staff on the need to maintain legible, contemporaneous records of pool water safety including chlorine levels and actions undertaken
• A review of the potential impact on pool water safety/monitoring procedures in the event of service disruption due to computer failure or similar events
• Considering measures to improve the use of shower before and after use of pool by customers
• Sharing the summary of this outbreak with the manufacturer of the colouring dye
• Enhanced vigilance and oversight when deviating from normal pool procedures, eg. when reducing chlorine levels or when auto-doser is switched off
• PHE has suggested a review of scientific evidence may be needed to determine how much chlorine is necessary to prevent the spread of norovirus in water.
• PHE is taking this up with the Pool Water Treatment Advisory Groupadvisory group (PWTAG), which publishes national guidance on pool water safety.

Medway Council has used the water dye at more than 10 events since 2015 but said it will not be using it again in the future.

A spokesman said: “Staff have good experience in using the colouring dye and manufacturer’s guidance is always followed.

“The normal operating procedure for Medway Council-run swimming pools includes a high but safe amount of chlorine – twice the national minimum guidance amount.

“At the time it was quickly determined that norovirus was the cause of the illness, and the pool and changing areas underwent an urgent deep clean as soon as reports of illness were received.

“As a precaution going forward, the council will no longer use dye in its pools.”

The council said they have already addressed PHE’s recommendations that staff undergo additional training on the need to maintain clear records and to consider ways to improve the use of showers before and after pool use by customers.

Cllr Howard Doe, portfolio holder for community services, said: “I would like to say again on behalf of the council that we are sorry to those who felt unwell after enjoying the pool party.”

A PHE spokesman said: "Our investigation revealed failures in pool maintenance procedures on the day of the outbreak, in relation to monitoring and recording of chlorine levels. Prompt actions by the Medway Council following reported illness minimised further risks to pool users. The report makes a number of recommendations to reduce the risk of such outbreaks occurring in the future.

“Our investigations also found that a lot of people do not take a shower before and after using the pool. Pool users are recommended to use the shower facilities before and after using the pool to minimise the health risks to themselves and other pool users.

“It’s very important that anyone suffering from diarrhoeal symptoms do not attend school or childcare settings for 48 hours after recovery. They should also avoid activities like swimming as this can be a way of passing the illness onto others.

“Anyone who is concerned about vomiting and diarrhoea symptoms should phone NHS 111 or your GP for advice.”