Subacute (28-day) toxicity of furfural in Fischer 344 rats: a comparison of the oral and inhalation route

Abstract

The subacute oral and inhalation toxicity of furfural vapour was studied in Fischer 344 rats to investigate whether route-to-route extrapolation could be employed to derive the limit value for inhalation exposure from oral toxicity data. Groups of 5 rats per sex were treated by gavage daily for 28 days at dose levels of 6–192 mg/kg bw/day, or exposed by inhalation to concentrations of 20–1280 mg/m³ (6 h/day, 5 days/week) or 160–1280 mg/m³ (3 h/day, 5 days/week) for 28 days. Controls received vehicle (corn oil) or were exposed to clean air.

Daily oral treatment with the highest dose of furfural (initially 192 mg/kg bw/day, later reduced to 144 mg/kg bw/day and finally to 120 mg/kg bw/day) resulted in mortality, and in increases in absolute and relative kidney and liver weight in surviving females of this group. Exposure of rats by inhalation for 6 h/day, 5 days/week for 28 days induced mortality at concentrations of 640 mg/m³ and above within 1–8 days. At 640 mg/m³ (3 h/day) and at 320 mg/m³ (3 and 6 h/day) and below, however, exposure was tolerated without serious clinical effects. In contrast, histopathological nasal changes were seen even at the lowest concentration of 20 mg/m³. With increasing exposure concentration, the nasal effects increased in incidence and severity and also expanded from the anterior part to the posterior part, including the olfactory epithelium.

It was concluded that the no-observed-adverse-effect level (NOAEL) for oral toxicity was 96 mg/kg bw/day. The NOAEL for systemic inhalation toxicity was comparable, i.e. 92 mg/kg bw/day (corresponding to 320 mg/m³ (6 h/day) or 640 mg/m³ (3 h/day)) assuming 100% absorption. The presence of the histopathological nasal changes at the lowest tested concentration of 20 mg/m³ (corresponding to 6 mg/kg bw/day) proves that for locally acting substances like furfural extrapolation from the oral to the inhalation route is not valid.

Introduction

Furfural (CAS Reg. no. 98-01-1) is a colourless, readily volatile oily liquid, with a pungent aromatic odour. It is used in solvent extraction processes in the petroleum refining industry and has a wide variety of other uses such as a solvent, an ingredient of phenolic resins, chemical intermediate, weed killer, fungicide and also as a flavouring agent (Kirk-Othmer, 1984). Furfural as a natural volatile compound has been identified in many foods, e.g. fruits, vegetables, bread, and beverages such as cognac, rum, malt whiskey, port wine, and coffee (Maarsse and Visscher, 1989). At the workplace, furfural may enter the body by the respiratory as well as the percutaneous route, the absorbed portion of the vapour by the skin corresponding to about 20% of the amount absorbed by the lungs (Flek and Sedivic, 1978). The lungs will retain about 78% furfural of the inhaled amount (Flek and Sedivic, 1978). Following oral intake in rats, highest concentrations, proportional to dose, were found in liver and kidneys. The lowest concentration was observed in the brain (Nomeir et al., 1992). Furfural is cleared from the body by rapid liver metabolism and excretion. The biotransformation of furfural takes place in two ways: the major part is oxidised and conjugated with glycine to furoylglycine, the smaller part is condensated with acetic acid and also conjugated with glycine to 2-furanacryluric acid (Flek and Sedivic, 1978). The major excretion route is via the urine, whereas exhalation by expired air and faecal excretion are minor routes (Nomeir et al., 1992).

Various values for the acute inhalation toxicity of furfural have been reported in rats varying from 600 mg/m³ (4-h LC50; RTECS, 2002), 740 mg/m³ (1-h LC50; Gupta et al., 1991) to 4075 mg/m³ (1-h LC50; Terrill et al., 1989). In hamsters, furfural has a low acute inhalation toxicity; a 4-h LC50 value of 13 g/m³ was indicated (Kruysse, 1972). A 6-h LC50 value of about 1.5 g/m³ has been reported for dogs (RTECS, 2002). These differences may indicate that metabolism plays a role. The sensory irritation potential as measured by respiratory rate depression resulted in RD50 values of about 900 and 1100 mg/m³ in two strains of mice (Steinhagen and Barrow, 1984). Inhalation exposure of hamsters to 448 or 2165 mg/m³ furfural for 6 h/day, 5 days/week during 13 weeks produced local toxicity especially in the nose as indicated by concentration-related focal atrophy of the nasal olfactory epithelium. No such effects were observed at the lowest concentration of 77 mg/m³ (Feron et al., 1979). Inhalation exposure during 12 months to a concentration of 1550 mg/m³ which was lowered after 20 weeks to 970 mg/m³, also revealed irritation of the nasal olfactory mucosa in the same species (Feron and Kruysse, 1978). Exposure of workers to furfural concentrations exceeding the threshold limit value of 8 mg/m³ has been reported to produce respiratory tract and/or eye irritation (Apol and Lucas, 1975; Di Pede et al., 1991).

The oral LD50 value for rats was reported to be between 50 and 127 mg/kg bw (Castelli et al., 1967; RTECS, 2002). In mice, oral LD50 values were higher, i.e. between 418 and 500 mg/kg bw (RTECS, 2002). Oral exposure of F344/N rats to furfural at dose levels of 15–240 mg/kg bw/day by gavage for 16 days resulted in mortality in 8 out of 10 rats at the highest level tested. A similar treatment of B6C3F1 mice up to dose levels of 400 mg/kg bw/day did not result in mortality. In both species, no compound-related histopathological lesions were observed. Upon treatment of the same rat strain for 13 weeks, the main effects consisted of mortality at levels of 90 and 180 mg/kg bw/day. In addition, increased absolute and relative liver and kidney weights were seen at a level of 90 mg/kg bw/day (at 180 mg/kg bw/day most rats had died). In mice survival was reduced at dose levels of 600 and 1200 mg/kg bw/day; increased liver and kidney weights were observed at levels of 75 mg/kg bw/day and higher, although dose-response relationships were not obtained. In mice, centrilobular necrosis and multifocal inflammation of the liver was reported at levels of 150 mg/kg bw/day and higher. When tested for carcinogenicity by the same treatment, it was concluded that there was some evidence of carcinogenicity based on the occurrence of uncommon cholangiocarcinomas in 2 males and bile duct dysplasia with fibrosis in two other males at a level of 60 mg/kg bw/day. In mice, it was concluded that there was clear evidence of carcinogenic activity at a level of 175 mg/kg bw/day, based on increased incidences of hepatocellular adenomas and carcinomas (Irwin, 1990).

From these data it can be concluded that the main target following oral exposure to furfural appeared to be the liver, whereas only local (nasal) effects have been reported following inhalation exposure. In the process of risk assessment, however, it is common practice to derive exposure limits for inhalation exposure from oral toxicity data. One of the criteria to perform route-to-route extrapolation is the absence of any local effects, i.e. the critical target tissue should not be at the portal of entry (Pepelko, 1987; Dourson and Felter, 1997). The present study with furfural was conducted in order to establish whether it is scientifically sound to apply route-to-route extrapolation to derive limit values for inhalation exposure from oral toxicity data. In addition to groups exposed for 6 h/day, groups exposed for 3 h/day were inserted in the present study to investigate whether daily exposure for 3 h to a given concentration would result in similar effects as daily exposure for 6 h to half that concentration.