Smoke is visible for 30 miles away after arsonists torched 66,000 tonnes of straw in Cambridgeshire.
The series of stack fires is so big it is expected to burn for days along the A142 corridor between Witcham Toll and Mepal, near Ely.
But the exact number of stacks on fire is difficult to tell as they are very close together.
Several straw stacks totalling some 16,000 bales are on fire in the Mepal area. Several more, totalling 50,000 tonnes are on fire in the Witcham Toll area.
Fire crews were alerted to the fires on Tuesday (13 December).
Chris Parker, station manager for Cambridgeshire Fire and Rescue Service said: “These are very large fires and as with all smoke, it does have a degree of toxicity.”
Residents and motorists in the area are being advised to keep windows and doors closed and contact NHS Direct on 0845 4647.
Mr Parker said: “At the moment, the smoke is blowing across fields towards Witchford, but if the wind direction changes we will issue further advice.”
Much of the straw was believed destined for nearby Ely Power Station – the largest straw burning power station in the world.
It represents one third of the straw supplied annually to the power station, which burns some 200,000 tonnes a year.
Operated by EPR, the power station can also burn oilseed rape and miscanthus. Cereal straw to the plant is supplied by sister company Anglian Straw.
The Ely power station is highly efficient, generating steam at 540°C. Noted for its reliability, the power station achieves one of the highest load factors of any renewable energy plant.
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