Toxicity Levels To Humans During Acute Exposure To Hydrogen Fluoride D .

- 4 - B. ACUTE TOXICITY 1. Routes of Exposure and Metabolism E xposu r e : Hydrogen fluoride commonly exists in two forms, as a gas or as a liquid when it is called hydrofluoric acid. In the liquid form it may pose an inhalation problem only in solutions more than 50% HF at 19.5 C. At this temperature and concentration HF has a partial pressure of only 1.86 kilopascals (14 m m H g ) . H o w e v e r , a 70% solution at 27 C has a partial pressure of approximately 20.0 kilopascals (150 m m H g ) . In this respect HF resembles volatile organics 1 -. k e acetone, chloroform or carbon tetrachloride (Ma 1 9 6 3 ) . As a dilute liquid, the danger of hydrogen fluoride exposure is probably greatest through skin contact. Hydrofluoric acid displays a rapid and insidious penetrating action when it has contacted the skin of workers. The acid passes easily through pin holes in rubber gloves and may cause delayed agonizing burns of the nail bed. Third degree destruction of tissue can occur from skin contact with 5 0 - 7 0 % solution of HF. Pain is felt immediately. Weaker solutions of 25% may take some minutes to be noticed. Burns from solutions of 1-20% may not be noticed for several hours (Ha 1 9 6 5 ) . Combinations of skin splashing and inhalation have produced severe systemic poisoning and death (Di 1962, Yo 1975, Sch 1978, Bu 1973, Sh 1 9 7 4 ) . Exposure to hydrogen fluoride in the gaseous form largely occurs through inhalation. The gas has a marked affinity for water and will combine with mist or water vapour in the air as well as the m o i s t u r e of the respiratory tract and the eyes. Even at fairly low levels, hydrogen fluoride will combine with moisture on the human skin to produce a smarting sensation (Ma 1 9 3 4 ) . There appear to be significant species differences in the consequences of inhalation of hydrogen f l u o r i d e . This has most recently been highlighted by studies on the regional deposition of inhaled hydrogen fluoride in rats (Mo 1 9 8 2 ) . Citing the fact that highly water soluble gases such as ammonia and sulphur dioxide are deposited in the upper respiratory tract with efficiencies of 95% (Da 1963) and at 99.99% (Fr 1969) respectively, the authors suspected that they might find a similar situation with hydrogen fluoride.

I I I I - 5 - In their inhalation study with rats they did indeed observe a deposition efficiency for hydrogen fluoride of greater than 99.9% in the upper respiratory tract for airborne concentrations of up to 71.3 m g / m 3 (87 ppm) (TWA). The authors speculate that this regional deposition in the rat explains the signs of nasal irritation exhibited by rats exposed to H F . In a number of studies (Ro 1 9 6 3 , Di 1 9 7 1 , Wo 1976) mucoid discharge from the external n a r e s , sneezing and/or pawing of the nose have been commonly observed in rats exposed to HF. Work in other laboratories (Ro 1963) noted pathologic damage to the nasal epithelium under conditions of HF exposure but did not mention any pulmonary injury using 4100 m g / m 3 (5000 p p m ) and above in the rats exposed to HF during L C 5 Q d e t e r m i n a t i o n s . Morris and Smith (Mo 1982) believe that significant HF deposition in the upper respiratory tract of the rat protects the rat pulmonary t i s s u e . They observed 100% lethality in rats exposed to 156 m g / m 3 (190 p p m ) for 6 hours but no lung edema. It seems unlikely that all m a m m a l s possess this protective capacity in the upper respiratory tract. D i P a s q u a l e and Davis (Di 1971) reported the 5 minute LC50 for mice was 5120 m g / m 3 (6,247 p p m ) and W o h l s l a g e l et al (Wo 1976) reported the 60 minute LC50 for the mouse was 280 m g / m 3 (342 p p m ) . These values are a p p r o x i m a t e l y one third to one quarter of the comparable values for the rat reported by this g r o u p . M o r r i s and Smith (Ho 1982) suggest that the increased sensitivity of the m o u s e may be due to increased p e n e t r a t i o n of the inhaled HF to the lungs in this s p e c i e s , but they do not seem to have considered the fact that the difference in species body weight might account for the varying v a l u e s . A more convincing example of the probable absence of HF nasal scrubbing activity comes from the work of H a c h l e et al (Ma 1934) who noted massive pulmonary edema and haemorrhaging in rabbits and guinea pigs exposed to 1500 m g / m 3 (1833 p p m ) H F . These animal studies concerning regional d e p o s i t i o n have c o n s i d e r a b l e significance when considering HF exposure in man. Regional deposition of HF in man has not been investigated. H o w e v e r , human subjects exposed to 26 m g / m 3 (32 p p m ) (Ma 1934) experienced irritation of the larger a i r w a y s . If this represents a direct effect rather than a reflex r e s p o n s e , it suggests that the nasal cavity of the human subject is much less efficient than of the rat in scrubbing airborne hydrogen f l u o r i d e . This would not be unexpected in the light of the more complex

- 6 - structure of the nasal cavity of the rat compared to the human (Ba 1 9 7 7 ) . These relatively recent findings concerning the regional deposition of HF in the rat tend to suggest that the rat might not be an appropriate model upon which to base human inhalation exposure t o l e r a n c e s . The study by Morris and Smith (Mo 1982) suggested that HF deposited in the upper respiratory tract, where there is a rich v a s c u l a t u r e , could be a point of rapid systemic absorption. H o w e v e r , systemic absorption of fluoride after oral administration is also quite rapid (Wa 1954) and it is possible that ingestion of HF contaminated nasal mucosa is also a significant entry route in the systemic absorption of fluoride ion. T ransport : Regardless of the route of e x p o s u r e , fluoride ion is readily absorbed into the blood stream and is carried to all organs of the body Where it is known to e q u i l i b r i a t e very rapidly across biological membrances (Wa 1 9 5 4 ) . Significant d e p o s i t i o n s of fluoride occur in calcified tissue such as bone (Wa 1 9 7 8 ) . There is growing evidence that the fluoride ion carried in the human blood serum exists in two f o r m s , namely as an inorganic ion F- and in c o m b i n a t i o n with an organic molecule. The latter is a small'but apparently significant a m o u n t . The nature of the o r g a n o f l u o r i n e molecule is still under i n v e s t i g a t i o n (Be 1 9 8 1 , Mo 1 9 8 3 , Ta 1 9 6 8 ) . Enzyme Inhibition: The transformation of a portion of the circulating inorganic fluoride ion to an organic form is not the only metabolic change that is known to occur to circulating inorganic fluoride. Fluoride passing through the soft tissue organs has an adverse effect on many enzymes since it serves as a wide spectrum enzyme i n h i b i t o r , even at relatively low c o n c e n t r a t i o n s . This is thought to occur because it forms m e t a l - f l u o r i d e - p h o s p h a t e c o m p l e x e s that interfere with the activity of those enzymes which require a metal ion c o f a c t o r . In a d d i t i o n , fluoride may interact directly with the enzyme or the s u b s t r a t e . It is a general inhibitor of the energy production system of the cell (oxidative p h o s p h o r y l a t i o n ) . Despite its obvious importance in enzyme competency there is a paucity of information in the literature concerning the wide spectrum of fluoride morbidity which is undoubtedly related to the

- 7 - effect of fluoride on many enzymes in numerous organs (Wa 1 9 7 8 ) . While many of the enzyme activities which are compromised by fluoride ion are known (Bo 1945) the significance of this in terms of human health remains undocumented. Distribution: R e g a r d l e s s of what kind of inorganic fluoride is absorbed, fluoride ion is the major form in which fluoride is distributed in the body. Because of its similarity to the chloride molecule it is considered that fluoride d i s t r i b u t i o n is essentially similar to that of chloride but it may be quantitatively different in certain o r g a n s . The distribution of fluoride among the tissues of the body is c h a r a c t e r i s t i c a l l y simple. An important part goes to the bone for d e p o s i t i o n , most of the remainder goes to the kidney for excretion in the u r i n e . No soft tissue stores fluoride (Wa 1 9 5 8 ) , but the kidney shows a temporary retention. As much as 1% of an intravenous dose may be present per gram of kidney 90 minutes after administration. Fluoride is rapidly removed from the kidney within 24 hours after an intravenous dose and essentially all the fluoride remaining in the body is limited to the skeletal d e p o s i t . Since the thyroid is known to store the related halogen iodine, some studies have been directed to d e t e r m i n e if this gland also has the same capacity to store f l u o r i d e . At least one of these studies (Ha 1954) has shown that there is no storage of fluoride in the thyroid. Skeletal Deposition: Fluoride is a bone seeker. About half of a fluoride dose given to an animal not p r e v i o u s l y treated will be deposited in the s k e l e t o n . The amount of skeletal fluoride and, usually, the concentration of fluoride in the bone increases with increasing fluoride intake when fluoride is supplied r e g u l a r l y . The average human diet probably furnishes between 0.5 and 1.5 milligrams of fluoride daily and bone c o n c e n t r a t i o n s of between 41-82 mg/m (50-100 p p m ) have been cited as being a p p r o x i m a t e normal c o n c e n t r a t i o n s in skeletal tissue (Ho 1 9 6 5 ) . Studies carried out by Neumann (Ne 1957) indicated that bone could reach an equilibrium with a surrounding fluoride solution so that a steady state level of fluoride was achieved in the bone.

Wailace-Durbin (Wa 1954) found that a major part of the ultimate skeletal deposit was removed from the circulation in the first 2 hours following intravenous administration. Neumann et al (Ne 1950) also showed that fluoride is deposited in bone by a simple exchange reaction with hydroxyl ions. This is thought to occur because the fluoride ion F- is very similar in size to the hydroxyl ion OH- and can therefore substitute for it easily in biochemical reactions. Fluoride ion is not deposited permanently in the skeleton, it is mobilized slowly with a half-life of something over two y e a r s . Largent (La 1960) showed that following a period of measured intake of fluoride the ion could be detected in the urine and feces for some c o n s i d e r a b l e time thereafter. The amounts decreased with time at such a rate that the excretion of the extra fluoride deposit in the skeleton required approximately two years to be removed. Industrial workers in cryolite p l a n t s , even after terminating employment, have been shown to excrete more urinary fluoride than unexposed i n d i v i d u a l s . Excretion: The major route for fluoride removal from the body is in the u r i n e . The urine excretion of fluoride is fast. Ericsson (Er 1958) showed that when a normal man ingested a glass of tapwater containing one milligram of fluoride more than 20% of the dose appears in the urine within the first four h o u r s . Smith et al confirmed this finding using 6 young adults drinking water containing 1.5 m i l l i g r a m s (total) of fluoride (Figure 1) (Ho 1 9 6 5 ) . From the viewpoint of worker exposure to hydrogen fluoride on a daily basis, it is important to know if monitoring of the urine fluoride is a good indicator of hydrogen fluoride e x p o s u r e . A number of studies on industrially exposed p o p u l a t i o n s seem to indicate that urine fluoride levels are reasonably accurate indicators of the amount of hydrogen fluoride exposure (Zo.1977, Wh 1980, La 1960) (Table 2 ) . These findings have been supported by human feeding studies which have correlated fluoride ion intake with urinary fluoride excretion (Figure 2 ) . For man and a number of other species, the relationship appears to be linear and approximately 50% of that fluoride which is absorbed is excreted in the urine.

- 9 - Fecal excretion of fluoride is highly variable and probably is influenced more by the solubility of the fluoride consumed than by any other single factor. When the fluoride absorbed into the system is from a source of hydrogen f l u o r i d e , then it is plausible to assume that ingested mucosa from the nasal p a s s a g e s , which is contaminated with f l u o r i d e , will nearly all be absorbed systemically from the gut and very little will be excreted in the f e c e s . Fluoride can also be excreted in p e r s p i r a t i o n , but the amounts are probably s m a l l , except in extremely hot conditions where excessive sweating may o c c u r . C o n c e n t r a t i o n s reported in p e r s p i r a t i o n have ranged from 0.16-1.15 mg/m-3 (0.2-1.4 p p m ) . The p e r c e n t a g e of the intake excreted in sweat under high temperature conditions can range from 13 to nearly 5 0 % (He 1 9 4 5 ) .

- 10 - 2. Animal Studies Inhalation studies involving animals exposed to HF have utilized the rat, mouse, guinea pig, pigeon, rabbit, dog and monkey as test species. The first reported HF animal inhalation study was performed by Ronzani in 1909 (Ro 1 9 0 9 ) . Five guinea pigs and 5 rabbits died in 0.5 and 1.5 hours, respectively, while being exposed to HF at airborne levels of 541 mg/m 3 (660 p p m ) . Five guinea pigs and 5 rabbits exposed to HF at 180 m g / m 3 (220 ppm) died in 1.0 and 3.0 hours respectively. Signs of severe irritation and increasingly difficult breathing were observed from the outset of the experiment. Autopsy revealed ulcérations of the upper respiratory tract and of the cornea of the e y e s . At 41 m g / m 3 (50 p p m ) , 5 guinea pigs died in two hours while 5 rabbits displayed signs of severe physical distress after 3 hours. At 25 m g / m 3 (30 p p m ) guinea pigs died after one day, while rabbits exposed to the same levels were in such poor condition after 3 d a y s , the experiment was discontinued. Continuous exposure to 8.2 m g / m 3 (10 ppm) for 5 days was not fatal to either s p e c i e s . The guinea pigs showed laboured breathing and slight eye irritation. Fifteen rabbits, 21 guinea pigs and 4 pigeons were exposed to 8.2 m g / m 3 (10 p p m ) for two 3-hour periods per day over 31 d a y s . During this period, 2 r a b b i t s , 7 guinea pigs and one pigeon died. A u t o p s i e s revealed opacity of the corneas with u l c é r a t i o n s , lesions of the nasal mucous m e m b r a n e s , emphysematous lungs, b r o n c h o p h e u m o n i t i s and interstitial p n e u m o n i t i s . Similar, though less severe findings, were revealed by the autopsy of one of the surviving a n i m a l s . All surviving animals were severely anemic and had lost up to 23% of their original w e i g h t . After immunization against t y p h u s , surviving animals showed a marked decrease in the production of specific antibodies and had reduced resistance to bacterial infection in the lung. Further experiments using HF levels of 6.1 m g / m 3 , 4.1 m g / m 3 and 2.5 m g / m 3 (7.5, 5 and 3 ppm) established 2.5 m g / m 3 (3 p p m ) as the no-effect c o n c e n t r a t i o n . A 30-day exposure of 16 rabbits, 20 guinea pigs and 3 pigeons to HF at 2.5 m g / m 3 (3 ppm) produced no pathologic c h a n g e s . In 1934 Machle et al (Ma 1934) studied acute effects in rabbits and guinea pigs exposed to HF at concentrations of 23-8,000 m g / m 3 (29-9,760 p p m ) for periods ranging from 41 hours to 5 m i n u t e s . For each exposure 3 rabbits and 3

- 11 - I I 1 guinea pigs were used. All animals displayed evidence of respiratory tract and eye irritation at all c o n c e n t r a t i o n s , although signs were mild and slow to appear in animals exposed to 50 and 24 m g / m 3 (60 and 29 ppm) for 5 to 15 m i n u t e s . Slowing of respiratory rate was uniformly observed and was expecially noticeable in rabbits. The frequency of coughs and sneezes increased as the HF level increased. Considerable kidney damage was observed in one rabbit exposed 6 hours per day for a total of 41 hours at 25 rag/m' (30 p p m ) . All animals exposed to levels greater than 500 m g / m 3 (610 ppm) for 15 minutes or more appeared ill and weak. These signs increased in severity as the airborne HF level was increased. Edema of organs and tissues was observed in animals exposed at concentrations of 3,000 m g / m 3 (3,660 p p m ) or more for 5-15 m i n u t e s . No deaths occurred under the following exposure c o n d i t i o n s : 1,000 m g / m 3 3 (1,220 p p m ) for up to 30 m i n u t e s , 98 m g / m (120 p p m ) for 5 hours and 24 m g / m 3 (29 ppm) for 41 h o u r s . Surviving rabbits returned to normal appearance and activity in a few days to a few w e e k s , whereas guinea pigs tended towards delayed response and death between the fifth and tenth weeks following e x p o s u r e . The predominant lesions found in exposed animals were as f o l l o w s : * I I ' ' * * * * pulmonary h a e m o r r h a g e , c o n g e s t i o n , emphysema and edema with secondary infection in many cases hepatic congestion with evidence of p a r e n c h y m a l necrosis and fatty degeneration splenic congestion and edema m y o c a r d i a l c o n g e s t i o n , edema and n e c r o s i s corneal erosions and ulcérations of nasal turbinâtes in many animals exposed to higher concentrations Some of these changes were also common to control g r o u p s . The a u t h o r s were unable to determine to what extent these changes were due to infection, nutritional c a u s e s , dietary d e f i c i e n c i e s or spontaneous disease p r o c e s s e s . In 1935 M a c h l e and Kitzmiller (Ma 1935) reported the effects of airborne HF on 5 rabbits, 3 guinea pigs and 2 Rhesus monkeys exposed to levels of 15.2 m g / m 3 (18.5 p p m ) for 6-8 hours daily except w e e k e n d s , until a total of 309 hours of exposure had been accumulated. All animals survived 8 months after exposure was concluded, except for 2 guinea p i g s . There was no noticeable response following introduction of the animals into the exposure chamber except for occasional coughing by one monkey. All animals exhibited slight lacrimation. No evidence of injury to

- 12 - the corneas or nasal p a s s a g e s w a s observed. Exposed rabbits had significantly lower erythrocyte counts than controls. S i g n i f i c a n t p a t h o l o g i c f i n d i n g s w e r e l i m i t e d to the l u n g s , l i v e r a n d k i d n e y s a n d w e r e m a r k e d in t h e 2 guinea pigs that died during the study. The following p a t h o l o g i c c h a n g e s w e r e o b s e r v e d in t h e 2 g u i n e a p i g s t h a t died: * * * * large pulmonary haemorrhage thickening and sloughing off of bronchial e p i t h e l i urn congestion, fatty degeneration, necrosis and diffuse periportal fibrosis of the liver s p o t t y tubular n e c r o s i s of t h

Figure 1 Urinary excretion of ingested fluoride by six men. Figure 2 Relationship between absorption and urinary excretion of fluoride in man (from Hachle and Largent 1943). Figure 3 LC50 Hydrogen fluoride levels for rats, guinea pigs, mice, and monkey. Figure 4 Hydrogen Fluoride Lethality Data Not Presented as LC50 or LD50