be transported indoors on people and their clothing as well as on pets. Clothing that was previously hung outdoors to dry is another source of pollen. Studies have shown that indoor pollen concentrations can be quite high during the pollen production season (O'Rourke et al., 1990; Platts-Mills et al., 1987; Pollart et al., 1988). Reports of indoor pollen concentrations have ranged from 0 to 5.5 million pollen grains per gram of house dust (O'Rourke and Lebowitz, 1984). Elevated concentrations of pollen in indoor air are found under open window conditions (in one case 600 grains per cubic meter in a room) (O'Rourke et al., 1989). These high airborne pollen concentrations are rare, but 30 pollen grains per cubic meter are not uncommon. Since people spend a majority of their time indoors (NAS, 1981; Quackenboss et al., 1991a,b), over 20 pollen grains can be inhaled in a typical household each day (on average, over all seasons). During the spring, when average outdoor pollen concentrations approach 400 grains per day, indoor exposure can approach 40–80 grains per day (O'Rourke, 1989).
Indoor plants are commonly found in office or school environments and in the home. Most are grown for their green foliage and accommodate low light or a lack of direct sunlight. As such, most do not flower in these environments and therefore are not pollen sources. Indeed, most plants grown indoors are not highly allergenic. Nevertheless, as more plants are used indoors, especially in large numbers in office settings, those considered not allergenic or only slightly allergenic may need to be reexamined. For example, Axelsson and colleagues (1987b, 1991) report that the leaves of Ficus benjamina (weeping fig) can produce airborne IgE-mediated rhinitis and asthma. They estimate that the risk of sensitization among truly atopic individuals is 6 percent. Hausen and Schulz (1988) report on a woman who developed conjunctivitis, rhinitis, and asthma from the nectar secretions of an ornamental plant, Abutilon striatum (flowering maple). Ford and co-workers (1986) report the development of IgE antibodies to pollen allergens from Parietaria judaica, an outdoor allergenic member of the nettle family found in the Mediterranean region but not likely to be found indoors.
Most recent reports in the literature regarding allergic reactions to indoor plants involve contact dermatitis produced by airborne allergens. Plants provoking such reactions include Allium (garlic), chlorophora (iroki; Fernandez de Corres et al., 1984, 1985), Chrysanthemum , citrus, Coleus (Van Hecke et al., 1991), common ferns (Geller-Bernstein et al., 1987), Compositae (daisy), Frullania (Pecegueiro and Brandao, 1985), lichens, Lilium (lily), Pelargonium (geranium), Philodendron, Pinus (pine), Platycodon grandiflorum (balloon flower; Nagano et al., 1982), Primula (primrose), Typha latifolia (cattail), and Umbelliferae (family name for parsley, carrots, coriander, etc.). Because many of these studies report the effects of mixtures of houseplants and garden plants, it is difficult to determine which house or indoor plants are truly allergenic.