There has been concern about air quality in the US among environmental groups after a recent landfill fire.
The burning flames in Tulsa, Oklahoma were thought to be caused by a lightning strike and have been blowing out of control for over a day.
It is feared that the smoke could contain toxic vapours.
While preliminary air monitoring results have confirmed that there are no poisonous particles in the smoke, the Oklahoma Department of Environmental Quality (ODEQ) and the Environmental Protection Agency (EPA) said that it can still cause health problems for nearby residents.
These, they state, include irritation to the respiratory system and breathing difficulties for asthma sufferers.
EPA regional administrator Al Armendariz said: "Emergencies such as this one demonstrate the importance of the continued planning and co-operation between ODEQ and EPA."
The groups have advised local residents to stay indoors to avoid unnecessary smoke inhalation.
Out of control forest fires in Russia have also caused concern this week as dense clouds of smoke filled Moscow's skies.
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