This family consists of 95 species of trees in two genera, Alnus Mill. and Betula L. Most occur in northern temperate regions, with some extending the range to the Arctic. Alnus species on the American continent extend as far south as Peru.
Both genera provide useful timbers, much used in turnery, manufacture of broomhandles, and similar purposes. Birch wood is used for the manufacture of plywood and paper, particularly in Scandanavia.
A study of the antigenic proteins in the pollen of Betula, Alnus, and certain other genera has demonstrated a close relationship between the families Betulaceae, Carpinaceae, and Corylaceae (Brunner & Fairbrothers 1979).
The timber from several species presents a dermatological hazard to woodworkers, the skin reactions apparently being of an allergic nature. Immediate-type hypersensitivity reactions of the skin to certain fresh fruit and vegetables and also to the sap from Betula have been observed in patients with Betula pollinosis.
This genus accounts for about 35 species. The name alder is popularly applied to several species of Alnus as well as to some botanically unrelated trees and shrubs.
The timber from an unspecified alder species is said to have caused dermatitis in one man and three women who had been working with the wood for years (Brügel & Perutz 1927). Patch tests with different extracts and alder tannin were positive, and negative in controls.
This species is widespread throughout Europe, but is more common in western and central regions. Alder wood dermatitis has been reported on several occasions (Senear 1933, Genner & Bonnevie 1938, Weber 1953, Orsler 1973, Hausen 1981), but considering the frequency with which the wood is used, the lack of detailed reports is surprising.
Investigations by Dässler & Urzynicok (1958) led to the isolation of a tannin from the wood, but this produced no dermatitis either in control subjects or in those suffering from alleged alder wood dermatitis.
This genus accounts for about 60 species. The anemophilous pollen from the catkins of birch species is released in large amounts in the spring and can cause pollinosis (Wodehouse 1971). Apple fruit [Malus domestica (Suckow) Borkh., fam. Rosaceae], carrot root [Daucus carota L. subsp. sativus, fam. Umbelliferae], parsnip root [Pastinaca sativa L., fam. Umbelliferae], potato tuber [Solanum tuberosum L., fam. Solanaceae], and less often other vegetables produced immediate-type hypersensitivity reactions in persons with birch pollinosis (Hannuksela & Lahti 1977). In a retrospective study by Andersen & Løwenstein (1978), the coincidence of birch pollen allergy and clinically relevant positive prick test reaction to fresh peel from apple and potato was confirmed. No immunological identity could be demonstrated between birch pollen antigens and extracts of apple and potato. However, non-immunological affinity precipitates were observed between birch pollen antibodies and the apple and potato extracts.
Woods & Calnan (1976) reported a cabinet maker with dermatitis who showed positive patch test reactions to Swedish birch, deal (possibly Pinus L. sp., fam. Pinaceae), teak (Tectona grandis L.f., fam. Labiatae), and to lapachol (see Bignoniaceae) and anthothecol (see Khaya anthotheca DC., fam. Meliaceae).
The timber from Betula lutea is listed as being irritant (Orsler 1973). The twigs when crushed release methyl salicylate.
The fragrance raw material sweet birch oil (also known as black birch oil or birch bark oil) is prepared by distillation of the bark of this species. It consists almost wholly of methyl salicylate. Arctander (1960) believed that the sweet birch oil of commerce is adulterated synthetic methyl salicylate. Undiluted sweet birch oil applied to the skin of rabbits, mice, and swine proved to be irritant. However, at a concentration of 4% in petrolatum in a closed patch test for 48 hours, sweet birch oil was shown to be non-irritant when applied to human skin. No phototoxicity could be demonstrated with the undiluted oil on mice or swine, and no sensitisation could be demonstrated in 25 human volunteers subjected to a maximisation test with 4% sweet birch oil in petrolatum (Opdyke 1979). Patch tests with 2% methyl salicylate produced 3 positive reactions in 183 eczema patients (Rudner 1977).
The dried bark from this species has been applied as a counter irritant (Morton 1977).
Some persons may develop urticaria following ingestion of salicylates (Noid et al. 1974).
This species, because of its waterproof bark, has been used by North American Indians for making canoes.
Dermatitis caused by this species has been attributed to the fine powder beneath the bark which causes folliculitis (Senear 1933).
Birch tar oil (also known as Oleum Rusci, Oleum Betulae Albae, Oleum Betulae Empyreumaticum, Pix Liquida Betulae, etc.) is obtained by the destructive distillation of the wood and bark of this species and also of the closely related Betula pubescens Ehrh. This tar oil has been used in external preparations for treating eczema, psoriasis and other chronic skin diseases (Todd 1967). It may also be used as a fragrance raw material. Opdyke (1973) reported that no irritating, sensitising, nor phototoxic properties could be demonstrated when birch tar oil was applied to mouse, rabbit, or human skin. However, Schwartz (1934) notes that occasional individuals may develop hypersensitivity to this material. Birch bud oil is a fragrance raw material distilled from the leaf buds of B. alba (Arctander 1960).
The wood has been reported to cause dermatitis (Duke 1927), and this was confirmed by patch testing. Howes (1951) has also observed contact dermatitis from this species. Hanslian & Kadlec (1966) list the wood as being able to cause dermatitis.
Phototoxicity from birch juice (Espersen 1952) lacks substantiation.
Fresh sap and crushed leaf from this species were tested by the scratch chamber method in 117 atopic persons, 74 of whom were allergic and 43 non-allergic to birch pollen, and in 33 control patients. Immediate positive reactions to birch sap were seen in 39% and to leaf in 28% of the patients allergic to birch pollen, and in 1 of the birch pollen-negative patients, but in none of the control subjects. Birch leaves may cause contact urticaria in the Finnish sauna, where bath whisks made of birch sprigs are traditionally used (Lahti & Hannuksela 1980).