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API Publication 4613: A Method for Estimating Dermal Absorption from Liquid Surfaces – Technical Overview and Compliance Guidance (1994)
Understanding the API 4613 Approach for Assessing Dermal Exposure Risks to Workers
Scope and Purpose
API Publication 4613 (API Publ 4613-1994) provides a standardized methodology for estimating the amount of a chemical that is absorbed through the skin when a worker’s skin contacts a liquid surface. Developed by the American Petroleum Institute, this publication addresses a critical gap in occupational exposure assessment by offering a practical, semi-empirical approach to quantify dermal uptake without the need for extensive in vivo studies.
The method is applicable to liquid chemicals that may be encountered in industrial settings, particularly in the petroleum and petrochemical industries. It is intended for use by industrial hygienists, toxicologists, and safety professionals who need to assess the dermal route of exposure as part of a comprehensive risk assessment. The publication emphasizes the importance of considering both the duration of skin contact and the physicochemical properties of the chemical (e.g., molecular weight, octanol-water partition coefficient, and water solubility) to estimate absorption rates.
API 4613 serves as a screening tool that can help prioritize chemicals for more detailed dermal absorption studies or guide the selection of personal protective equipment (PPE). It has been referenced in various regulatory frameworks and industry guidelines, making it a foundational document in the field of dermal risk assessment.
Tip: API 4613 is best used as an initial screening method. For high‑risk chemicals or when more precise exposure data are needed, consider supplementing with in vitro or in vivo dermal absorption studies and following guidance from organizations such as the EPA or OECD.
Technical Requirements of the Method
Core Input Parameters
The API 4613 method relies on a limited set of input variables to estimate the mass absorbed per unit area of skin. The key parameters include:
- Exposure duration (t): The length of time the skin is in contact with the liquid surface (minutes or hours).
- Skin surface area exposed (A): Typically expressed in cm², with default values provided for common body parts (e.g., hand, forearm).
- Chemical concentration in the liquid (C): In mg/cm³ or mg/L, representing the amount of chemical per volume of liquid.
- Absorption fraction (AF): A dimensionless factor (0 to 1) that accounts for the portion of the chemical on the skin that is actually absorbed. The publication provides default AF values for different chemical classes, based on literature and expert judgment.
- Flux or permeation coefficient (Kp): In cm/h, derived from the chemical’s molecular weight (MW) and octanol‑water partition coefficient (log Kow) using an empirical correlation presented in the document.
The absorbed dose (D) is calculated as:
D = (C × Kp × t × A × AF) / BW
where BW is the body weight (kg) used to normalize the dose for comparison with acceptable daily intake or occupational exposure limits.
Data Table: Typical Input Values from API 4613
| Parameter | Units | Example Values | Remarks |
|---|---|---|---|
| Skin area (one hand) | cm² | 445 | Default for male workers |
| Skin area (forearm) | cm² | 850 | Includes hand and forearm |
| Exposure duration | h | 0.5 – 8 | Based on task or shift |
| Absorption fraction (AF) | dimensionless | 0.1 – 1.0 | Default values per chemical class (e.g., phenols, hydrocarbons) |
| Permeation coefficient (Kp) | cm/h | 10⁻⁴ – 10⁻² | Estimated from MW and log Kow |
| Body weight (BW) | kg | 70 | Default adult male |
Important: The default absorption fractions and Kp correlations in API 4613 were developed primarily for liquid hydrocarbons and similar organic compounds. Application to chemicals with very different physicochemical properties (e.g., surfactants, particulate slurries) may require validation or adjustment.
Implementation Highlights
To apply the API 4613 method effectively, the following workflow is recommended:
- Identify scenario: Characterize the task, worker activity, and potential for skin contact with liquid surfaces (e.g., immersion, splashing, wiping).
- Obtain chemical properties: Collect molecular weight, log Kow, and water solubility from reliable databases (e.g., EPI Suite, HSDB).
- Compute Kp: Use the API 4613 correlation: log Kp = -2.80 + 0.66 log Kow – 0.0055 MW. If MW > 600 or log Kow > 6, the correlation may be less reliable; consult alternative models.
- Select absorption fraction (AF): Refer to Table 2 of the publication, which provides AF values for 15 chemical classes (e.g., alcohols, ketones, aliphatic hydrocarbons). For chemicals not listed, a default AF of 1.0 may be used as a conservative assumption.
- Estimate exposure parameters: Determine the likely contact time and skin area using standard ergonomic data (e.g., hand surface area ~ 445 cm²).
- Calculate dose: Apply the formula to obtain absorbed dose (mg/kg/day) and compare with the appropriate limit (e.g., oral reference dose, occupational exposure limit).
API 4613 does not require specialized software and can be implemented in a spreadsheet. The method is designed to be transparent and conservative, providing a reasonable worst‑case estimate when input values are chosen judiciously.
Best practice: Where possible, validate the API 4613 prediction with available in vivo or in vitro data for the specific chemical. The method is most robust when used as part of a weight‑of‑evidence approach.
Compliance Notes and Considerations
While API 4613 is a publication and not a formal regulatory standard, it has been adopted by many organizations as a de facto method for estimating dermal absorption in the absence of specific data. Key compliance points include:
- OSHA/EPA alignment: In the United States, EPA’s Occupational and Residential Exposure guidelines reference API 4613 as an acceptable method for assessing dermal exposure to liquids. When conducting exposure assessments for compliance with OSHA permissible exposure limits (PELs) or EPA risk assessments, using API 4613 can satisfy data call‑ins if no other information is available.
- International use: The method is cited in the European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) Technical Report No. 116 and has been used in REACH dermal risk assessments. However, regulators may require a more refined approach (e.g., OECD Test Guideline 428 for in vitro skin absorption) for high‑volume chemicals or those with high hazard potential.
- Limitations: API 4613 does not account for skin hydration, temperature, occlusion, or the presence of multiple chemicals (mixtures). It assumes a homogeneous liquid film and does not address the influence of evaporation or matrix effects. Users must document these assumptions in their risk assessment.
- Reevaluation: Since 1994, newer dermal absorption models have been developed (e.g., the Potts‑Guy correlation, QSAR models). It is advisable to cross‑check API 4613 outputs with these updated models, especially when the estimated dose is close to a safety limit.
Caution: Using API 4613 without considering its limitations can lead to significant underestimation of dermal exposure. The method should never be used for highly volatile chemicals (which may evaporate before significant absorption) or for corrosives where immediate skin damage is the primary hazard.
In summary, API Publ 4613-1994 remains a valuable tool in the industrial hygienist’s toolkit. It provides a straightforward, reproducible framework for estimating dermal absorption that is widely accepted in occupational health practice. When applied with awareness of its assumptions and supplemented with relevant chemical‑specific data, it supports sound decision‑making regarding worker protection and exposure control.
Q: Is API Publication 4613 the same as ASTM E 2230 or other dermal absorption methods?
A: No. API 4613 is specific to liquid surface contact and uses a correlation based on molecular weight and log Kow. ASTM E 2230 (now withdrawn) and OECD TG 428 are in vitro methods that measure flux through skin samples. The API method is a predictive, less resource‑intensive approach suited for screening.
A: No. API 4613 is specific to liquid surface contact and uses a correlation based on molecular weight and log Kow. ASTM E 2230 (now withdrawn) and OECD TG 428 are in vitro methods that measure flux through skin samples. The API method is a predictive, less resource‑intensive approach suited for screening.
Q: Can I use API 4613 for chemicals that are solids or powders?
A: The publication is designed for liquids that form a discrete surface layer. For solids, the dermal absorption pathway is highly dependent on particle size and dissolution in sweat, which is not captured by the model. Alternative guidance, such as the EPA’s Dermal Exposure to Solids Guidance, should be used instead.
A: The publication is designed for liquids that form a discrete surface layer. For solids, the dermal absorption pathway is highly dependent on particle size and dissolution in sweat, which is not captured by the model. Alternative guidance, such as the EPA’s Dermal Exposure to Solids Guidance, should be used instead.
Q: Where can I obtain the full text of API Publ 4613-1994?
A: The publication is available from the American Petroleum Institute (API) on their standards shop. Some university and corporate libraries may also have copies. Since it is a 1994 document, it may not be updated, but the methodology remains current.
A: The publication is available from the American Petroleum Institute (API) on their standards shop. Some university and corporate libraries may also have copies. Since it is a 1994 document, it may not be updated, but the methodology remains current.
Q: Does the method require human data for validation?
A: No, but if human in vivo data exist (e.g., from controlled skin patch studies), they can be used to refine the absorption fraction (AF). In the absence of data, the publication provides default AF values that are considered protective for most hydrocarbons.
A: No, but if human in vivo data exist (e.g., from controlled skin patch studies), they can be used to refine the absorption fraction (AF). In the absence of data, the publication provides default AF values that are considered protective for most hydrocarbons.