Worldwide, the total area for greenhouse crop production was estimated by Lenteren & Woets (1988) at 150,000 ha. Only about 18% of this area is covered by traditional glass greenhouse coverings; the remaining area consists of polyethylene or acrylic-covered structures. Some greenhouses have sophisticated environmental control computers that regulate many facets of greenhouse operations, including environmental factors such as temperature, light, and humidification. Others have only minimal environmental controls e.g. heaters. The remaining greenhouse area consists largely of polyethylene-covered structures which do not have environmental controls.