Air quality tests were carried out following fears of a radiation leak after a fire at a factory which manufactures smoke detectors.
The blaze at BRK on the Waterwells business park in Gloucester started on Thursday night.
The smoke and fire detectors contain a small amount of radioactive material but monitoring outside the factory has found no trace of contamination.
The cause of the fire is still being investigated.
A team from the Environment Agency have been scanning piles of the burnt-out alarms for radioactivity and the area has been declared safe.
It is believed 350,000 smoke alarms were destroyed.
BRK will now have to hire specialist decontamination contractors to demolish the site.
All approach roads to the area have now been re-opened but access to The Quadrant remains closed.
Roads to the Waterwells business park in Quedgeley were shut and junction 12 of the M5 was closed after the blaze broke out on Thursday evening.
At the time people were asked to stay in buildings and keep windows shut amid fears the smoke may have been toxic.
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