STEEL company Corus has been given six months to prevent red dust from affecting residents living close to its Teesside plant.
An investigation was launched on November 8 after complaints from residents.
The Environment Agency inspected the plant and discovered the red powder was deposits of rust from iron ore.
An improvement notice has been issued with a list of 11 actions to carry out within a six-month deadline.
These include looking into whether the dust can be removed before the iron ore is brought to the site.
The agency has also suggested improvements to the systems used for handling, storing and moving the raw material.
Corus will be required to set up monitoring systems to prevent any further repeats of the incident.
Howard Stuttard, industry team leader for the agency, said: "We took this incident very seriously, even though we were sure fairly early on that there would be no effect on people's health.
"We understand the concerns of the community, and the improvement notice we served will make Corus reassess how it handles iron ore in the future to prevent this happening again."
A spokeswoman for the company said action had already been taken to reduce the amount of dust coming from the plant. She said: "We will be doing everything in our power to take on board these improvements within the six months."
The Environment Agency said further action could be taken against Corus if the 11 actions were not met in the time given.
She said: "If the company had shown complete willingness to address the problems, but had not completed them in time, a short extension could be handed out.
"In extreme situations, we have the power to force them to do what we ask."
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