Air pollutants
The most common and serious forms of greenhouse pollution are combustion gases generated by faulty heat exchangers, dirty fuel openings and incomplete fuel combustion. Well-sealed, energy efficient greenhouses have added to the problem by reducing outside air exchanges. In low concentrations, carbon monoxide (CO) can cause headaches and dizziness for workers and above 50 ppm (0.005%), injury and death can occur. Leaks of fuel such as propane and methane must be fairly large to be hazardous for human health, but even small leaks can adversely affect plants. Similarly, ethylene is only dangerous to humans at high concentrations, but ethylene levels of less than 0.05 ppm can make tomato leaves bend downward (epinasty). With chronic exposure, stems may thicken, branching may increase, and flower buds may abort or develop into malformed fruit. Under chronic low exposures, however, symptoms may be hard to recognize, especially if clean air grown plants are not available for comparison. Diagnosis is also difficult because of the time lag between the period of ethylene exposure and the time damage is noted. The safest practice is to maintain proper ventilation, even at the expense of energy conservation, and observe plants closely for signs of damage when heaters first come on in the fall and during periods of unusually cold weather in the winter.
Problems are most likely in grower installations or when the heater is not designed for year-round greenhouses. For example, use of animal confinement and tobacco transplant house heaters is not recommended. Problems arise when inadequate combustion air within the greenhouse prevents complete combustion and proper venting of combustion gases. These gases can leak into the greenhouse from the draft diverter on the heater. The easiest way to ensure an adequate supply of oxygen for combustion is to run a pipe or flexible hose from outside the house to the burner enclosure. One square inch of louvered air inlet area from outside per 2500 BTU furnace capacity is necessary to provide adequate fresh air [28]. The outside entrance should be screened to protect it from blockage. Proper venting is also essential to prevent exhaust gases from being drawn back into the house.
Proper maintenance also prevents problems. Adjustment and checking for gas leaks is best done by professionals before the start of the heating season. Maintenance should include cleaning the unit heater and fuel orifice at least twice a year and regularly inspecting the flame for changes in appearance. Propane flames should have a small yellow tip while natural gas flames should be soft blue, with a well-defined inner cone.