Section 4: Air Pollutants
Various air pollutants or potentially harmful compounds can occur in the greenhouse atmosphere. Carbon monoxide (CO) is dangerous to people. It may be generated by malfunctioning heaters, vehicles, arc welders other combustion engine machinery. Unit heaters without internal heat exchangers should be avoided as they may emit CO into the greenhouse if not functioning properly. Additionally, poorly or improperly maintained and vented unit heaters may result in CO entering the greenhouse. A CO concentration of 50 ppm is generally considered to be the upper limit for human safety (the OSHA PEL limit).
In addition to generating CO, ethylene may be generated by such devices as malfunctioning heaters, vehicles, arc welders and other combustion engine machinery. Ethylene (C2H4) is a gas that can be highly injurious to plants. Very low concentrations (as low as 0.05 ppm) of ethylene can cause plant damage. Typical plant symptoms of exposure to ethylene include epinasty (malformed leaves that curl or corkscrew often in a downward direction), "sleepy" flowers (flowers appear wilted and curled), and abscission or abortion of flowers and fruits. Sources of ethylene should be avoid and excluded from the greenhouse environment, and unit heaters should be checked periodically to be sure that they are functioning correctly (follow the manufacturer’s recommendations on proper maintenance and operation) and the exhaust fumes properly vented to outside of the greenhouse. Ripening fruits and vegetables also produce ethylene so these products should not be stored in coolers, growth chambers or other enclosed spaces with plant materials.
If CO2 burners are used in greenhouses, low sulfur fuels should be used to avoid producing injurious levels of SO2.
Herbicides can be damaging to plants even at very low concentrations. Caution should be taken to insure that herbicides applied out-of-doors do not drift (and are not pulled in by fans) into the greenhouse. Some herbicides are labeled for greenhouse use, and herbicide labels should always be followed. However, even when used in greenhouse according to label directions, problems can occur. For example, if herbicides are sprayed onto active heating pipes, the high temperature can cause volatilization of phytotoxic components even though this would not occur under normal application conditions.
Numerous chemicals including paints and cleaning materials may release potentially damaging volatile chemicals and should be used in or around greenhouses with caution.
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