Hydrotropes are used as coupling agents to solubilize the water insoluble and often incompatible functional ingredients of household and institutional cleaning products and personal care products. These hydrotropes are not surfactants but are used to solubilize complex formulations in water. They function to stabilize solutions, modify viscosity and cloud-point, limit low temperature phase separation and reduce foam. This assessment considers salts of toluene, xylene and cumene sulfonates. Hydrotropes are amphiphilic substances composed of both a hydrophilic and a hydrophobic functional group. The hydrophobic part of the molecule is a benzene substituted apolar segment. The hydrophilic, polar segment is an anionic sulfonate group accompanied by a counter ion (i.e., ammonium, calcium, potassium or sodium).
Hydrotropes are produced by sulfonation of an aromatic hydrocarbon solvent (i.e., toluene, xylene or cumene). The resulting aromatic sulfonic acid is neutralized using an appropriate base (e.g., sodium hydroxide) to produce the sulfonate or hydrotrope. The hydrotropes are pure substances but are produced and transported in either aqueous solutions, typically at a 30-60% level of activity, or in granular solids typically at 90-95% level of activity. The other components of granular solids include sodium sulphate and water.
The consumption of hydrotropes in laundry detergent and household cleaning product applications is 17,000 tonnes in 2002 according to a survey of hydrotrope producers and formulators that are HERA members in Europe. This HERA-reported consumption is believed to account for at least 80% of total hydrotrope tonnages used in HERA applications in Europe (the basis of the 80% default can be obtained from HERA). Important HERA application products are household laundry and cleaning products, such as laundry powders and liquids, liquid fabric conditioners, liquid and powder laundry bleach additives, hand dishwashing liquid, machine dishwashing liquid, liquid and gel toilet cleaners, and liquid, powder, gel and spray surface cleaners. This HERA assessment is based on the 17,000 tonnes consumption figure.
For the purposes of this assessment, it is assumed that the entire 17,000 tonnes/year volume is in products that are ultimately released down-the-drain, and following wastewater treatment, the ingredient may be released into the environment.
The present environmental risk assessment of hydrotropes is based on the revised HERA methodology document (HERA, 2002), which in its turn is based on the revised EU Technical Guidance Document (TGD, 2003). It makes use of the EUSES 2.02 programme (EUSES, 2005) which is now compatible with the HERA detergent scenario. Hydrotrope concentrations modelled in the various environmental compartments were compared with extrapolations of the available eco-toxicity data or modelled eco-toxicity values leading to PNEC values protective of each compartment.
" The modelled hydrotropes concentration in raw sewage is 1.16 mg/l. Approximately 87% of hydrotropes are removed in activated sludge sewage treatment plants (STP) yielding a modelled effluent concentration of 0.147 mg/L. Hydrotropes are readily biodegraded under aerobic conditions.
" Dilution of the STP effluent in the receiving waters results in a local estimated concentration of 0.0205 mg/L hydrotropes. The corresponding regional surface water estimate is 0.006 mg/L.
" Predictions for the other local PECs are: 0.002 mg/kg in dry sewage sludge; 2.07E-06 mg/kg wet weight in soil; and 0.0161 mg/kg wet weight in freshwater sediments.
" Fugacity modelling supports the prediction that 99+% of hydrotropes will reside in the water compartment and that negligible amounts will end up in soil, sediments, air or biota. Hydrotropes have a very low measured octanol-water partition coefficient and will therefore not bioaccumulate.
" Aquatic ecotoxicity data are available and well documented for representatives of the hydrotropes category. The aquatic PNEC value is 0.23 mg/L
" Corresponding PNEC values for other environmental compartments are: 0.180 mg/kg for freshwater sediments, 0.027 mg/kg for soils, and 160 mg/L for STPs.
" The risk characterisation, as expressed by the PEC/PNEC ratio, was far below 1 for all environmental compartments. It was therefore concluded that the use of hydrotropes in household laundry and cleaning products does not pose a risk for the environment. Further, the margin of safety would accommodate any additional hydrotrope volumes/uses not accounted for in the HERA assessment.
Human health assessment
The presence of hydrotropes in many commonly used household detergent and cleaning products gives rise to a variety of possible consumer contact scenarios including direct and indirect skin contact, inhalation, and oral ingestion derived either from hand washing of clothes and dishes, residues deposited on dishes and clothes, from accidental product ingestion, or indirectly from drinking water. A standard risk assessment methodology was used to derive Margins of Exposure.
" The consumer aggregate exposure from direct and indirect skin contact as well as from inhalation and from oral route in drinking water and dishware results in an estimated total body burden of 1.42 µg/kg bw/day. The consumer aggregate external exposure from direct and indirect skin contact is 87 µg/kg bw/day using worst case assumptions.
" Toxicological data are available and well documented for representative tolune, xylene and cumene sulfonates (including sodium, potassium, ammonium and calcium salts). These data demonstrate that hydrotropes have a low order of acute toxicity by all relevant routes (LC50s range from 100s to 1000s mg/kg), are not genotoxic in vitro or in vivo, show no evidence of a carcinogenic response (or any other systemic toxicity) in 2-year dermal exposure studies, and failed to induce developmental, teratogenic or fertility (sex organ) effects.
" Adverse effects after repeated long term dosing of hydrotropes to animals included epidermal hyperplasia at the site of application in dermal studies, and decreased relative spleen weight in females in oral studies. The critical adverse effect and corresponding systemic NOAEL is 763 mg a.i./kg bw based upon decreased relative spleen weight in female rats in a 90-day oral study. The NOAEL for local effects, based on epidermal hyperplasia at the site of application, was 440 mg a.i./kg bw for mice in 90-day dermal studies.
" Comparison of the aggregate internal consumer exposure to hydrotropes (1.42 µg/kg bw) with the systemic NOAEL (763 mg a.i./kg bw) results in an estimated Margin of Exposure (MOE) of >500,000. Comparison of the aggregate external consumer exposure (87 µg/kg bw) with the epidermal hyperplasia NOAEL (440 mg a.i/kg bw) results in an estimated MOE of >5,000. Both of these MOEs are very large MOE; large enough to account for the inherent uncertainty and variability of the hazard database and inter species and intra species extrapolations (which are usually conventionally estimated at a factor of 100.
" Hydrotropes can be classified as a negligible-to-slight irritant to skin and a slight-to-moderate irritant to eyes. The irritation potential of aqueous solutions of hydrotropes depends on concentration, and the irritation is lessened with rinsing. Hydrotropes are not considered to be skin sensitizers.
" In view of the database on toxic effects, the low exposure values calculated and the resulting large Margin of Exposure, it can be concluded that use of hydrotropes in household laundry and cleaning products raises no safety concerns for the consumers.