They can be heard from Almondbury to Quarmby, and from Jackson Bridge to Linthwaite.

For more than 30 years the public warning sirens at Huddersfield chemical giant Syngenta’s base at the Manufacturing Centre on Leeds Road have sounded through the town once a year.

This year’s testing will happen on Thursday, June 9, between 10am and noon.

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And with the company celebrating 100 years of manufacturing in Huddersfield it is planning to have a special guest to press the button and sound the alarm.

Community relations manager Carl Sykes said: “We upgraded our siren system about three years ago and with certain weather conditions and wind direction, we know that the sirens can be heard from afar.”

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The firm tests its sirens on low volume every month and gives the full volume sound test once a year.

“We often get asked about what action people should take if they hear them. People should be reassured that whilst the sirens may be heard, they should not cause alarm if people have not had a letter from us.

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“For those residents who live close to the site and do have a letter, the advice is simple. Go in, stay in and tune in if the alarm is ever raised outside of our testing programme.”

Every year the firm writes to 9,000 households locally in its public information zone and includes a copy of its community safety plan, giving advice on what to do if they ever sound for real.