Sodium tripolyphosphate (STPP) is a solid inorganic compound used in a large variety of household cleaning products, mainly as a builder, but also in human foodstuffs, animal feeds, industrial cleaning processes and ceramics manufacture.
STPP is widely used in regular and compact laundry detergents (powder, liquid, gel, tablets), automatic dishwashing detergents (powder, liquid, gel, tablets), toilet cleaners, and surface cleaners, and provides a number of functions including sequestration of “water hardness” enabling surfactants to function effectively, pH buffering, dirt emulsification and prevention of deposition, hydrolysis of grease, and dissolving-dispersing dirt particles. In the year 2000, the total consumption of STPP in these applications was estimated to be approximately 300 000 tonnes in Western Europe and this is estimated to represent 90-95% of STPP use in Europe.
Environmental risk assessment
Due to its physico-chemical properties, STPP is not distributed or transported to the atmosphere, and thus is not expected to end up in soil via atmospheric deposition. Because it is very water-soluble, it is not significantly transferred to sewage sludge, and therefore to soil by sludge spreading. No environmental risk related to STPP use in detergents is indicated in soil or air.
As an ingredient of household cleaning products, STPP present in domestic waste waters is mainly discharged to the aquatic compartment, directly, via waste water treatment plants, via septic tanks, infiltration or other autonomous waste water systems.
As STPP is an inorganic substance, biodegradation studies are not applicable. However, STPP can be hydrolysed, finally to orthophosphate, which can be assimilated by algae and/or by micro-organisms. STPP thus ends up being assimilated into the natural phosphorus cycle.
Reliable published studies confirm biochemical understanding, showing that STPP is progressively hydrolysed by biochemical activity in contact with waste waters (in sewerage pipes and within sewage works) and also in the natural aquatic environment.
This information enabled the calculation of “worst case” PEC (Predicted Environmental Concentrations) using the EUSES model and the HERA detergent scenario. A default regional release of 10 % was applied instead of the 7 % regional release indicated in the HERA detergent scenario.
Reliable acute aquatic ecotoxicity studies are available which show that STPP is not toxic to aquatic organisms: all EC/LC50 are above 100 mg/l (daphnia, fish, algae). Because of this, and because of the only temporary presence of STPP in the aquatic environment (due to hydrolysis), no studies have been carried out to date concerning the chronic effects of STPP on these aquatic organisms. PNEC (Predicted No Effect Concentrations) were therefore calculated for the aquatic environment and sediments on the basis of the acute aquatic ecotoxicity results.
PEC/PNEC ratio >1 were obtained for STPP in the local water and sediment compartments (2.5 and 3.18 respectively). STPP has been used for many years. It is an authorised food and drug additive. It did not show any acute toxicity effect to the aquatic organisms tested. It is anticipated that the PEC/PNEC ratios >1 obtained for the local water and sediment scenarios are the consequence of the use of the 1,000 Assessment Factor in the calculation.
To refine the aquatic PNEC of STPP, CEEP (Centre Européen d’Etudes des Polyphosphates) has planned acute toxicity tests on Daphnia magna and Ceriodaphnia dubia, as well as a chronic toxicity test on the most sensitive species if necessary.
Concerning the possible environmental impact of STPP-based orthophosphate (PO43-), a preliminary risk characterisation based on simplified but conservative assumptions was conducted. A PEC/PNEC ratio < 1 was obtained for the local water compartment, indicating that orthophosphate resulting from the hydrolysis of STPP does not present a risk for the aquatic environment.
The eutrophication of surface waters due to nutrient enrichment is not addressed in this document because a PNEC cannot be defined for such effects, which depend on many factors varying spatially and temporally (temperature, light, concentrations of phosphates and of other nutrients, activity of grazer population …).
Human Health risk assessment
Consumers can be exposed to STPP from household cleaning products by all routes, skin contact, oral ingestion, or by inhalation. Using scenarios relevant to consumer uses, the total potential exposure was estimated to be 33 µg/kg/day.
The toxicological database shows that STPP has a low acute toxicity by ingestion and dermal application. At the maximum attainable concentration of STPP that could be technically generated, no significant clinical signs were observed except reactions consistent with exposure to an irritant dust.
STPP was not found irritating to the intact skin or to the eyes when tested neat or in aqueous solutions. Experimental data that were available with detergent formulations containing STPP showed no skin contact sensitisation potential under typical use conditions. Furthermore, there are also no reports of skin sensitisation occurrence associated with STPP exposure in consumers.
Based on literature data, STPP is not considered to be mutagenic or genotoxic. The oral long-term toxicity study did not show any evidence of a carcinogenic potential of STPP in a chronic study in rats. There was no evidence of adverse reproductive or developmental effects in various species at the doses tested.
No repeated dose toxicity studies were available in animals for the dermal route, or by inhalation. However, repeated dose toxicity studies in rats by the oral route showed that STPP at high doses induced retarded growth, anaemia and renal calcification. In a 2-year study, no toxic effect was observed at the doses of up to 0.5%, which was used to estimate a systemic NOEL of 225 mg/kg/day. The effects induced by STPP were similar to those reported for other condensed inorganic phosphates.
As Phosphorus accounts for 25.3% of the STPP molecule, the total intake of Phosphorus through the use of STPP in detergents (8.4 µg P/kg/day) is minor when compared to the estimated typical dietary intake of Phosphorus in European countries (ca. 24 mg/kg/day) and to the Maximum Tolerable Daily intake established for total Phosphorus intake from all dietary sources (70 mg/kg/day).
Comparison of the total combined estimated systemic consumer exposure to STPP with the systemic NOEL results in a margin of exposure of 6 800 that can be considered large enough to conclude that STPP is of low concern for consumer use in household detergents, taking into account inherent uncertainties, variability of the database and extrapolations.