Dibutyl hydrogen phosphate

Toxicological Summary

Workers - Hazard via inhalation route

Systemic effects

Long term exposure

Hazard assessment conclusion:

DNEL (Derived No Effect Level)

Value:

4.4 mg/m³

Most sensitive endpoint:

repeated dose toxicity

Route of original study:

Oral

DNEL related information

DNEL derivation method:

ECHA REACH Guidance

Overall assessment factor (AF):

5

Dose descriptor starting point:

NOAEL

Value:

8.9 mg/kg bw/day

Modified dose descriptor starting point:

NOAEC

Value:

22 mg/m³

Explanation for the modification of the dose descriptor starting point:

For the inhalation route there is no animal study available. Therefore, oral rat data is used to calculate a corresponding air concentration for humans and a route-to-route extrapolation for systemic effects is necessary to derive the correct starting point. In the case of oral-to-inhalation the inclusion of a default factor of 2 is recommended according to chapter R.8.4.2 of the ECHA Guidance on Information Requirements and Chemical Safety Assessment, chapter R.8: Characterisation of dose [concentration]-response for human health (version 2.1, November 2012) in the absence of route specific information. No default factor is included here for oral-to-inhalation extrapolation, since for single or multiple oral doses a bioavailability of 100 % was experimentally proven (Gatz, 1992). According to Figure R. 8-3 in the ECHA Guidance additional correction is needed for scaling issues: Corrected inhalation NOAEC = oral NOAEL * 1/0.38 m³ per kg and day * 6.7 m³/10 m³ * 1.4 (for differences in experimental/human exposure conditions, i.e. 7 days/week in animal study versus 5 days/week for workers, see Appendix R.8-6 of ECHA Guidance R8).

Based on the oral NOAEL of 8.9 mg/kg bw/day for systemic toxicity obtained in a study on rats with chronic treatment the starting point is calculated with 22 mg/m³.

AF for dose response relationship:

1

Justification:

Since the starting point for the DNEL calculation is a NOAEL according to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor for dose response relationship is 1.

AF for differences in duration of exposure:

1

Justification:

For differences in experimental exposure duration (= chronic) and the duration of exposure for the population and scenario under consideration (= chronic) according to Table R.8-5 of ECHA Guidance R.8 no assessment factor is considered.

AF for interspecies differences (allometric scaling):

1

Justification:

Allometric scaling is already included in the route-to-route extrapolation for dose descriptor calculation as described in ECHA Guidance R.8.

AF for other interspecies differences:

1

Justification:

A factor of 2.5 for remaining interspecies differences is suggested in ECHA guidance R.8. However, toxicological information for rats, mice and rabbits is available for tributyl phosphate and the most sensitive endpoint and species was taken to define the starting point for DNEL calculation. Based on long term studies on rats and mice the most sensitive endpoint was hyperplasia and papilloma in the bladder of rats, and the rat was found as most sensitive species. DNEL calculation was thus derived on this study and no factor for interspecies differences is needed.

AF for intraspecies differences:

5

Justification:

According to chapter R.8.4.3.1 of ECHA guidance R.8 the default assessment factor to be applied for intraspecies differences in workers is 5.

AF for the quality of the whole database:

1

Justification:

The default assessment factor is used as suggested by ECHA Guidance R.8, based on the available toxicological studies for tributyl phosphate.

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

carcinogenicity

DNEL related information

Local effects

Long term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

DNEL related information

Workers - Hazard via dermal route

Systemic effects

Long term exposure

Hazard assessment conclusion:

DNEL (Derived No Effect Level)

Value:

0.6 mg/kg bw/day

Most sensitive endpoint:

repeated dose toxicity

Route of original study:

Oral

DNEL related information

DNEL derivation method:

ECHA REACH Guidance

Overall assessment factor (AF):

20

Dose descriptor starting point:

NOAEL

Value:

8.9 mg/kg bw/day

Modified dose descriptor starting point:

NOAEL

Value:

12.5 mg/kg bw/day

Explanation for the modification of the dose descriptor starting point:

For the dermal route there is no animal study available. Therefore, oral rat data is used to calculate a corresponding dermal exposure dose for humans. On the assumption that, in general, dermal absorption will not be higher than oral absorption, no default factor (i.e. factor 1) should be introduced when performing oral-to-dermal extrapolation (ECHA Guidance R.8, chapter 8.4.2). For tributyl phosphate absorption via dermal route is experimentally proven to be between 40 to 56%, opposed to 100% via the oral route (Graz, 1992). Thus, no modification of the dose descriptor starting point is warranted. However, since there are differences in experimental/human exposure conditions (7 days/week in animal study versus 5 days/week for workers) adaption with a factor of 1.4 is appropriate (Appendix R.8-6 of ECHA Guidance R8). Based on the oral NOAEL of 8.9 mg/kg bw/day for systemic toxicity obtained in a study on rats with chronic treatment, the starting point is calculated with 12.5 mg/kg bw/day.

AF for dose response relationship:

1

Justification:

Since the starting point for the DNEL calculation is a NOAEL according to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor for dose response relationship is 1.

AF for differences in duration of exposure:

1

Justification:

For differences in experimental exposure duration (= chronic) and the duration of exposure for the population and scenario under consideration (= chronic) according to Table R.8-5 of ECHA Guidance R.8 no factor is considered.

AF for interspecies differences (allometric scaling):

4

Justification:

According to Table R.8-3 of ECHA Guidance R.8 the allometric scaling factor for the rat when compared with humans is 4.

AF for other interspecies differences:

1

Justification:

A factor of 2.5 for remaining interspecies differences is suggested in ECHA guidance R.8. However, toxicological information for rats, mice and rabbits is available for tributyl phosphate and the most sensitive endpoint and species was taken to define the starting point for DNEL calculation. Based on long term studies on rats and mice the most sensitive endpoint was hyperplasia and papilloma in the bladder of rats, and the rat was found as most sensitive species. DNEL calculation was thus derived on this study and no factor for interspecies differences is needed.

AF for intraspecies differences:

5

Justification:

According to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor to be applied for intraspecies differences in workers is 5.

AF for the quality of the whole database:

1

Justification:

The default assessment factor is used as suggested by ECHA Guidance R.8, based on the available toxicological studies for tributyl phosphate.

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

carcinogenicity

DNEL related information

Local effects

Long term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

Workers - Hazard for the eyes

Local effects

Hazard assessment conclusion:

medium hazard (no threshold derived)

Additional information - workers

The acute toxicity of dibutyl hydrogen phosphate was determined with an acute oral LD50 and dermal LD50 of > 2000 mg/kg bw.

The results of an in vitro skin corrosion study led to a classification with Skin Corr. 1B (H314). In a study on rabbits dibutyl phosphate was shown to be severely irritating to the eye (Eye Damage 1).  Dibutyl hydrogen phosphate is not a skin sensitizer in Guinea pigs.

Dibutyl hydrogen phosphate was investigated in the Salmonella/microsome test (Ames test). No evidence of mutagenic activity of dibutyl hydrogen phosphate was seen (with and without mutagenic activation). Additionally, the substance was evaluated negative in a cytogenetic assay in mammalian cells and also in a CHO/HGPRT test in mammalian cells. In an in vivo micronucleus assay in mice no indication for a mutagenic effect was found as well.

For dibutyl hydrogen phosphate a combined repeated dose and reproductive/developmental toxicity screening test according OECD TG 422 is available. Ten rats per sex and group were administered doses of 0, 30, 100, 300 or 1000 mg/kg bw/day of the test substance per gavage. Administration periods were 44 days for males. Females were exposed for 14 days before mating, during mating and pregnancy, and up to day 3 of lactation. Males were sacrificed on day 45 and females on day 4 of lactation.
Transient red urine and a decrease in food consumption was observed at ≥ 100 mg/kg bw/day. Three males and 2 females died in the 1000 mg/kg bw/day group. Histopathology showed epithelial hyperplasia accompanied by degeneration and ulceration of the urinary bladder mucosa in males and females at ≥ 100 mg/kg bw/day. At higher doses epithelial hyperplasia and hyperkeratosis of the forestomach (some with erosion and ulceration in the gastric mucosa) were noted in both sexes (≥ 300 mg/kg bw/day). Increase of absolute and relative liver weight and hepatocellular swelling was seen in females of the 1000 mg/kg bw/day group. The NOEL in this study is considered to be 30 mg/kg bw/day for both sexes, based on the most sensitive target organ, the bladder.

Dibutyl hydrogen phosphate is the main metabolite of tributyl phosphate. Tributyl phosphate undergoes a rapid metabolization to dibutyl hydrogen phosphate by the microsomal enzyme complex in the liver. The elimination occurs independently from the application form via urine (50 - 70 %), followed by exhalation (7 - 10%) and feces (4 - 6%) (BUA report no 108). Therefore, and based on the fact that dibutyl and tributyl phosphate have the same main target organ and both lead to hyperplasia in the rat bladder, a read across with tributyl phosphate (CAS No. 126-73-8) as a surrogate for dibutyl hydrogen phosphate (CAS 107-66-4) is considered adequate. (see ‘Justification for read-across’ attached to the Dossier)

The toxicology of tributyl phosphate is well investigated in many studies and species. The main target organ of tributyl phosphate in repeated oral dose toxicity studies, comparable to dibutyl phosphate, is the rat bladder. Hyperplasia and papilloma were induced by tributyl phosphate and chronic oral exposure led to malign bladder tumors only in rats. No histopathologic observations are reported for the urinary bladder in a chronic study of tributyl phosphate in mice up to the highest dose tested (3500 ppm). The NOEL in the rat carcinogenicity study with chronic exposure over 24 months was determined with 8.9 mg/kg bw/day for male rats and 11.6 mg/kg bw/day for female rats.

A mechanistic feeding study with tributyl phosphate was conducted in male rats to clarify the mode of action of bladder carcinogenicity in rats. In this study urinary bladder proliferation occurred in the 700 (53 mg/kg bw(day) and 3000 ppm (230 mg/kg bw/day) groups of tributyl phosphate, with the changes being more severe at the higher dose. No changes were observed in the bladder at 200 ppm (15 mg/kg bw/day). The proliferative response was completely reversible in the group fed 3000 ppm tributyl phosphate for 10 weeks followed by 10 weeks of recovery. Submucosal fibrosis was present at 21 weeks of the total study, representing repair of ulceration and inflammation produced by tributyl phosphate administration.
One possible hypothesis behind the origin of tributyl phosphate induced urothelial necrosis and ulceration, might be that they are the consequence of secondary responses to one or more of the metabolites of tributyl phosphate since little tributyl phosphate is excreted unchanged in the urine. Repeated cellular damage induced by irritating metabolites followed by chronic compensatory repair eventually leads to the formation of metaplastic and neoplastic cells. The indirect mechanism for this cell transformation is supported by the lack of genotoxicity, lack of increased mitotic activity (labeling index), and full reversal of the hyperplasia and proliferation after 10 weeks treatment followed by 10 weeks of recovery. These data support a low human health risk from exposure to tributyl phosphate.

Tributyl phosphate is thus classified with Carc Cat 2 (H351). This classification was confirmed by CoRAP 2012, in that ‘Based upon several studies epithelial hyperplasia of the urinary bladder was evident and urinary bladder papillomas or carcinomas also appeared. As all the mutagenicity tests were negative, tributyl phosphate may be considered a non-genotoxic carcinogenic substance.’ (Substance Evaluation Conclusion document of September 6, 2013)
The classification with Carc Cat 2 (H351) is assigned to dibutyl hydrogen phosphate.

Since the rat is the most sensitive species for the toxicological effects of tributyl phosphate and bladder carcinogenicity is the most critical and sensitive effect, the rat carcinogenicity study is used as basis for DNEL derivation regarding systemic effects after inhalation, dermal and oral exposure to dibutyl hydrogen phosphate. A DNEL and not a DMEL was derived since the mode of action of bladder tumor induction of tributyl phosphate is a non-genotoxic one. Both, tributyl and dibutyl hydrogen phosphate do not exert a mutagenic potential.

The DNEL for dibutyl phosphate is thus derived from the rat carcinogenicity feeding study with chronic exposure to tributyl phosphate over 24 months and a NOEL of 8.9 mg/kg bw/day for systemic toxicity.

General Population - Hazard via inhalation route

Systemic effects

Long term exposure

Hazard assessment conclusion:

DNEL (Derived No Effect Level)

Value:

0.77 mg/m³

Most sensitive endpoint:

repeated dose toxicity

Route of original study:

Oral

DNEL related information

DNEL derivation method:

ECHA REACH Guidance

Overall assessment factor (AF):

10

Dose descriptor starting point:

NOAEL

Value:

8.9 mg/kg bw/day

Modified dose descriptor starting point:

NOAEC

Value:

7.74 mg/m³

Explanation for the modification of the dose descriptor starting point:

For the inhalation route there is no animal study available. Therefore, oral rat data is used to calculate a corresponding air concentration for humans and a route-to-route extrapolation for systemic effects is necessary to derive the correct starting point. In the case of oral-to-inhalation the inclusion of a default factor of 2 is recommended according to chapter R.8.4.2 of the ECHA Guidance on Information Requirements and Chemical Safety Assessment, chapter R.8: Characterisation of dose [concentration]-response for human health (version 2.1, November 2012) in the absence of route specific information. No default factor is included here for oral-to-inhalation extrapolation, since for single or multiple oral doses a bioavailability of 100 % was experimentally proven (Gatz, 1992). According to Figure R. 8-3 of ECHA Guidance R.8 additional correction is needed for scaling issues: Corrected inhalation NOAEC = oral NOAEL * 1/1.15 m³ per kg and day (based on the oral NOAEL of 8.9 mg/kg bw/day for systemic toxicity). Thus, a starting point of 7.74 mg/m³ is calculated.

AF for dose response relationship:

1

Justification:

As the starting point for the DNEL calculation is a NOAEL according to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor for dose response relationship is 1.

AF for differences in duration of exposure:

1

Justification:

For differences in experimental exposure duration (= chronic) and the duration of exposure for the population and scenario under consideration (= chronic) according to Table R.8-5 of ECHA Guidance R.8 no assessment factor is considered.

AF for interspecies differences (allometric scaling):

1

Justification:

rat versus human: According to table R.8-4 in chapter R.8 of the ECHA guidance document (version 2.1, November 2012) the AF of 4 is already included in the route to route extrapolation.

AF for other interspecies differences:

1

Justification:

A factor of 2.5 for remaining interspecies differences is suggested in ECHA Guidance R.8. However, toxicological information for rats, mice and rabbits is available for tributyl phosphate and the most sensitive endpoint and species was taken to define the starting point for DNEL calculation. Based on long term studies on rats and mice the most sensitive endpoint was hyperplasia and papilloma in the bladder of rats, and the rat was found as most sensitive species. DNEL calculation was thus derived on this study and no factor for interspecies differences is needed.

AF for intraspecies differences:

10

Justification:

According to chapter R.8.4.3.1 of ECHA guidance R.8 the default assessment factor to be applied for intraspecies differences in the general population is 10.

AF for the quality of the whole database:

1

Justification:

The default assessment factor is used as suggested by ECHA Guidance R.8, based on the available toxicological studies for tributyl phosphate.

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

carcinogenicity

DNEL related information

Local effects

Long term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

DNEL related information

General Population - Hazard via dermal route

Systemic effects

Long term exposure

Hazard assessment conclusion:

DNEL (Derived No Effect Level)

Value:

0.22 mg/kg bw/day

Most sensitive endpoint:

repeated dose toxicity

Route of original study:

Oral

DNEL related information

DNEL derivation method:

ECHA REACH Guidance

Overall assessment factor (AF):

40

Dose descriptor starting point:

NOAEL

Value:

8.9 mg/kg bw/day

Modified dose descriptor starting point:

NOAEL

Value:

8.9 mg/kg bw/day

Explanation for the modification of the dose descriptor starting point:

For the dermal route there is no animal study available. Therefore, oral rat data are used to calculate a corresponding dermal exposure dose for humans. On the assumption that, in general, dermal absorption will not be higher than oral absorption, no default factor (i.e. factor 1) should be introduced when performing oral-to-dermal extrapolation (ECHA Guidance R.8, chapter 8.4.2). For tributyl phosphate absorption via dermal route is experimentally proven to be between 40 to 56%, opposed to 100% via the oral route (Graz, 1992). No differences in experimental/human exposure conditions were considered (7 days/week in the animal study and for the general population). Thus, no modification of the dose descriptor starting point is warranted.

AF for dose response relationship:

1

Justification:

As the starting point for the DNEL calculation is a NOAEL according to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor for dose response relationship is 1.

AF for differences in duration of exposure:

1

Justification:

For differences in experimental exposure duration (= chronic) and the duration of exposure for the population and scenario under consideration (= chronic) according to Table R.8-5 of ECHA Guidance R.8 no assessment factor is considered.

AF for interspecies differences (allometric scaling):

4

Justification:

According to Table R.8-3 of ECHA Guidance R.8 the allometric scaling factor for the rat when compared with humans is 4.

AF for other interspecies differences:

1

Justification:

A factor of 2.5 for remaining interspecies differences is suggested in ECHA Guidance R.8.
However, toxicological information for rats, mice and rabbits is available for tributyl phosphate and the most sensitive endpoint and species was taken to define the starting point for DNEL calculation. Based on long term studies on rats and mice the most sensitive endpoint was hyperplasia and papilloma in the bladder of rats, and the rat was found as most sensitive species. DNEL calculation was thus derived on this study and no factor for interspecies differences is needed.

AF for intraspecies differences:

10

Justification:

According to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor to be applied for intraspecies differences in the general population is 10.

AF for the quality of the whole database:

1

Justification:

The default assessment factor is used as suggested by ECHA Guidance R.8, based on the available toxicological studies for dibutyl hydrogen phosphate.

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

carcinogenicity

DNEL related information

Local effects

Long term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

skin irritation/corrosion

General Population - Hazard via oral route

Systemic effects

Long term exposure

Hazard assessment conclusion:

DNEL (Derived No Effect Level)

Value:

0.22 mg/kg bw/day

Most sensitive endpoint:

repeated dose toxicity

Route of original study:

Oral

DNEL related information

DNEL derivation method:

ECHA REACH Guidance

Overall assessment factor (AF):

40

Dose descriptor starting point:

NOAEL

Value:

8.9 mg/kg bw/day

Modified dose descriptor starting point:

NOAEL

Value:

8.9 mg/kg bw/day

Explanation for the modification of the dose descriptor starting point:

Oral data from the rat are used to decide on a corresponding oral dose for humans. Therefore, a route-to-route extrapolation is not necessary and the NOAEL from the rat study is used as starting point.

AF for dose response relationship:

1

Justification:

As the starting point for the DNEL calculation is a NOAEL according to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor for dose response relationship is 1.

AF for differences in duration of exposure:

1

Justification:

For differences in experimental exposure duration (= chronic) and the duration of exposure for the population and scenario under consideration (= chronic) according to Table R.8-5 of ECHA Guidance R.8 no assessment factor is considered.

AF for interspecies differences (allometric scaling):

4

Justification:

According to Table R.8-3 of ECHA Guidance R.8 the allometric scaling factor for the rat when compared with humans is 4.

AF for other interspecies differences:

1

Justification:

A factor of 2.5 for remaining interspecies differences is suggested in ECHA Guidance R.8.
However, toxicological information for rats, mice and rabbits is available for tributyl phosphate and the most sensitive endpoint and species was taken to define the starting point for DNEL calculation. Based on long term studies on rats and mice the most sensitive endpoint was hyperplasia and papilloma in the bladder of rats, and the rat was found as most sensitive species. DNEL calculation was thus derived on this study and no factor for interspecies differences is needed.

AF for intraspecies differences:

10

Justification:

According to chapter R.8.4.3.1 of ECHA Guidance R.8 the default assessment factor to be applied for intraspecies differences in the general population is 10.

AF for the quality of the whole database:

1

Justification:

The default assessment factor is used as suggested by ECHA Guidance R.8, based on the available toxicological studies for dibutyl hydrogen phosphate.

Acute/short term exposure

Hazard assessment conclusion:

medium hazard (no threshold derived)

Most sensitive endpoint:

carcinogenicity

DNEL related information

General Population - Hazard for the eyes

Local effects

Hazard assessment conclusion:

medium hazard (no threshold derived)

Additional information - General Population

The acute toxicity of dibutyl hydrogen phosphate was determined with an acute oral LD50 and dermal LD50 of > 2000 mg/kg bw.

The results of an in vitro skin corrosion study led to a classification with Skin Corr. 1B (H314). In a study on rabbits dibutyl phosphate was shown to be severely irritating to the eye (Eye Damage 1). Dibutyl hydrogen phosphate is not a skin sensitizer in Guinea pigs.

Dibutyl hydrogen phosphate was investigated in the Salmonella/microsome test (Ames test). No evidence of mutagenic activity of dibutyl hydrogen phosphate was seen (with and without mutagenic activation). Additionally, the substance was evaluated negative in a cytogenetic assay in mammalian cells and also in a CHO/HGPRT test in mammalian cells. In an in vivo micronucleus assay in mice no indication for a mutagenic effect was found as well.

For dibutyl hydrogen phosphate a combined repeated dose and reproductive/developmental toxicity screening test according OECD TG 422 is available. Ten rats per sex and group were administered doses of 0, 30, 100, 300 or 1000 mg/kg bw/day of the test substance per gavage. Administration periods were 44 days for males. Females were exposed for 14 days before mating, during mating and pregnancy, and up to day 3 of lactation. Males were sacrificed on day 45 and females on day 4 of lactation.
Transient red urine and a decrease in food consumption was observed at ≥ 100 mg/kg bw/day. Three males and 2 females died in the 1000 mg/kg bw/day group. Histopathology showed epithelial hyperplasia accompanied by degeneration and ulceration of the urinary bladder mucosa in males and females at ≥ 100 mg/kg bw/day. At higher doses epithelial hyperplasia and hyperkeratosis of the forestomach (some with erosion and ulceration in the gastric mucosa) were noted in both sexes (≥ 300 mg/kg bw/day). Increase of absolute and relative liver weight and hepatocellular swelling was seen in females of the 1000 mg/kg bw/day group. The NOEL in this study is considered to be 30 mg/kg bw/day for both sexes, based on the most sensitive target organ, the bladder.

Dibutyl hydrogen phosphate is the main metabolite of tributyl phosphate. Tributyl phosphate undergoes a rapid metabolization to dibutyl hydrogen phosphate by the microsomal enzyme complex in the liver. The elimination occurs independently from the application form via urine (50 - 70 %), followed by exhalation (7 - 10%) and feces (4 - 6%) (BUA report no 108). Therefore, and based on the fact that dibutyl and tributyl phosphate have the same main target organ and both lead to hyperplasia in the rat bladder, a read across with tributyl phosphate (CAS No. 126-73-8) as a surrogate for dibutyl hydrogen phosphate (CAS 107-66-4) is considered adequate.

The toxicology of tributyl phosphate is well investigated in many studies and species. The main target organ of tributyl phosphate in repeated oral dose toxicity studies, comparable to dibutyl phosphate, is the rat bladder. Hyperplasia and papilloma were induced by tributyl phosphate in rats and mice, whereas chronic oral exposure led to malign bladder tumors only in rats. This was shown in carcinogenicity studies with tributyl phosphate in rats and mice. The NOEL in the rat carcinogenicity study with chronic exposure over 24 months was determined with 8.9 mg/kg bw/day for male rats and 11.6 mg/kg bw/day for female rats.

A mechanistic feeding study with tributyl phosphate was conducted in male rats to clarify the mode of action of bladder carcinogenicity in rats. In this study urinary bladder proliferation occurred in the 700 (53 mg/kg bw(day) and 3000 ppm (230 mg/kg bw/day) groups of tributyl phosphate, with the changes being more severe at the higher dose. No changes were observed in the bladder at 200 ppm (15 mg/kg bw/day). The proliferative response was completely reversible in the group fed 3000 ppm tributyl phosphate for 10 weeks followed by 10 weeks of recovery. Submucosal fibrosis was present at 21 weeks of the total study, representing repair of ulceration and inflammation produced by tributyl phosphate administration.
One possible hypothesis behind the origin of tributyl phosphate induced urothelial necrosis and ulceration, might be that they are the consequence of secondary responses to one or more of the metabolites of tributyl phosphate since little tributyl phosphate is excreted unchanged in the urine. Repeated cellular damage induced by irritating metabolites followed by chronic compensatory repair eventually leads to the formation of metaplastic and neoplastic cells. The indirect mechanism for this cell transformation is supported by the lack of genotoxicity, lack of increased mitotic activity (labeling index), and full reversal of the hyperplasia and proliferation after 10 weeks treatment followed by 10 weeks of recovery. These data support a low human health risk from exposure to tributyl phosphate.

Since the rat is the most sensitive species for the toxicological effects of tributyl phosphate and bladder carcinogenicity is the most critical and sensitive effect, the rat carcinogenicity study is used as basis for DNEL derivation regarding systemic effects after inhalation, dermal and oral exposure to dibutyl hydrogen phosphate. A DNEL and not a DMEL was derived since the mode of action of bladder tumor induction of tributyl phosphate is a non-genotoxic one. Both, tributyl and dibutyl hydrogen phosphate do not exert a mutagenic potential.

The DNEL for dibutyl phosphate is thus derived from the rat carcinogenicity feeding study with chronic exposure to tributyl phosphate over 24 months and a NOEL of 8.9 mg/kg bw/day for systemic toxicity.