While occasionally using red cedar to smoke food is not harmful to health there are many reasons why using red cedar is sometimes said to be unsafe.
Treated Wood
Some concern for using red cedar comes from the fear of using treated wood. Treated wood used in building projects contains chemicals that make it unsafe for use in cooking. Heat causes the chemicals to transfer into the food and can result in serious illness. When deciding to use red cedar in cooking make sure the wood is untreated. “Food grade” cedar planks can be purchased from gourmet cooking retailers.
Resins
Concern for health also comes from the resins in cedar wood. Unlike hardwoods such as walnut and oak, or fruitwoods such as cherry and apple used in roasting, cedar is a softwood and sometimes accused of harboring large amounts of harmful resins. This is a myth; cedar does not harbor excessive or harmful resins and is prized in building for its lack of resin. Some dislike the taste of cedar-smoked meat and others claim illness from cedar resins in the food, but these cases are not supported by medical findings.
Eastern Red Cedar
Confusion between species of cedar is a serious concern for those roasting with cedar wood. Eastern red cedar (Juniperus virginian) is an evergreen plant toxic to humans and not safe for consumption. While called cedar, it is actually a juniper with a cedar-like scent that is used in aromatic oils. When cooking, be sure that the wood you use is western red cedar (Thuja plicata).
Cedar Dust
The United States Occupational Health and Safety Administration (OSHA) identifies western red cedar dust inhalation as a cause of respiratory problems such as rhinitis and asthma. Dermatitis and eye irritation have also been linked to acute exposure to western red cedar dust. However, these effects are the result of wood dust, a byproduct of manufacturing cedar in lumberyards and sawmills. Smoking or roasting food with western red cedar for short periods will not produce these effects.