Fifty species in seven genera are native to North temperate and sub-tropical regions, south to India, IndoChina and the Andes.
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25 species are native to eastern North America. The trees (hickory) are cultivated for their wood and for the edible fruit e.g. pecans. Some species yield juglone (Thomson 1971).
Contact dermatitis occurring in a nut merchant from oil of pecan was briefly noted by Swinny (1951).
The pollen of pecan (Carya pecan) and other Carya spp. can cause hayfever (Wodehouse 1971).
Fifteen species are native to the Mediterranean regions, eastern Asia, IndoChina and the Andes. Many species are cultivated for their fruit. The wood is valuable. The seeds yield oil.
A naphthaquinone, juglone, derived from J. regia, J. nigra, J. cinerea (butternut) and from some other plants of this family is present in the green parts as a glycoside or in reduced form and has been identified in water run-off from the trees (Thomson 1971).
The pollen of some species is suspected of causing hayfever (Wodehouse 1971).
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The juice of the nut shells stains the skin dark brown. Schwartz (1931) stated that occupational dermatitis from this source was not known to occur. A single case of dermatitis of the finger-webs from occupational exposure was of irritant nature (Siegel 1954).
This west Asian tree is widely cultivated and naturalized. Oil of Walnut is used for varnish, paint and perfumery (Greenberg and Lester 1954).
In Chinese traditional medicine, the oil from walnut seeds (nuts) is used as an application to several kinds of skin diseases, including eczema, chancre, and favus. The oil may also be applied to the hair as a pomade. A dye obtained from the pericarp of the fruit or from the bark or root of the tree may also be used to darken the hair. Juglans sieboldiana Maxim. is similarly used and not distinguished from Juglans regia in its medicinal uses (Stuart 1911). In NW Moroccan traditional medicine, an infusion of the leaf is taken orally for eczema, cutaneous infections and rheumatism; and the dried root bark is used externally for tooth care and gingivitis (Merzouki et al. 2000).
Contact-sensitivity to the wood was observed in five cases (Schürkämper 1972) and a case of contact dermatitis was reported by Schleicher (1974). A cabinet-maker who had extensive dermatitis showed a positive patch test reaction to walnut. He also showed positive patch test reactions to other woods, viz. teak (Tectona grandis L. f., fam. Labiatae) and Turreanthus and to other plants and allergens so that the significance of the positive reaction to walnut was considered doubtful. This patient has been receiving treatment for three years by systemic steroid administration for his illness which was considered to be polyarteritis with pulmonary involvement. He changed his occupation and made a complete recovery from the dermatitis (Calnan 1970, Woods and Calnan 1976). It seems possible that this case was an instance of occupational skin and lung wood toxicity or allergy.
The leaves, wood, bark and nut shells yield juglone, a naphthoquinone. According to Telegina (1965), juglone (5-oxy-1,4-naphthoquinone) is not irritant or sensitising. However, irritancy of juglone was reported by Auyone et al. (1968); and contact senstiivity to juglone was reported by Barniske (1957). Attempts to use juglone as a topical sunscreen were abandoned because commerically available juglone was found to yield a chromatographic fraction capable of causing allergic contact dermatitis; this fraction was unstable under ordinary conditions of storage (Runge 1972). Juglone was found to be a strong sensitiser for guinea pigs (Hausen 1974). Cross-sensitivity to quinones was reviewed by Mitchell (1975).
Ingestion of English walnuts has been correlated with recurrent oral ulceration (Editorial 1974).