Eye Irritation Test: A Scientific and Ethical Approach to Ocular Safety Assessment

The eye is one of the body’s most sensitive and vulnerable organs when exposed to chemicals. Ocular tissues such as the cornea, iris, and conjunctiva can suffer irreversible damage upon chemical contact, leading to serious consequences that may even result in permanent vision loss. Therefore, determining the acute irritant and corrosive effects that chemicals, pesticides, cleaning products, cosmetic ingredients, and industrial formulations can have on the eye is an indispensable requirement for both user safety and GHS classification. The eye irritation test forms the scientific basis for this assessment. Our laboratory conducts eye irritation and corrosion tests in accordance with the latest OECD guidelines, under GLP principles, and with full adherence to the 3R approach.

The Scientific Basis and Importance of Eye Irritation

Eye irritation is a local, typically reversible inflammatory response resulting from a substance’s contact with the eye. Clinically, it manifests as conjunctival redness (erythema), conjunctival edema (chemosis), corneal opacity, iris damage, tearing, pain, and a burning sensation. Eye corrosion, on the other hand, is a more severe condition in which a substance causes irreversible damage to ocular tissues; it is characterized by corneal ulceration, necrosis, perforation, and permanent vision loss.
Distinguishing between these two conditions is of critical importance; because a substance with corrosion potential poses an unacceptable risk to user safety and is classified in the highest hazard category according to GHS criteria.

The risk of eye irritation is not limited to industrial chemicals. Cleaning products, personal care products, cosmetics, agricultural pesticides, and even some food additives used in daily life can come into contact with the eyes. Children, in particular, are at higher risk from such substances due to their habit of rubbing their eyes.

Eye irritation data is directly used to determine the hazard statements and pictograms used in product labeling, to prepare safety data sheets (SDS), to establish emergency response protocols, and to define recommendations for personal protective equipment.
Furthermore, these data are a fundamental requirement for regulatory authorities’ chemical safety assessments and product authorization processes.

OECD TG 405: Classical In Vivo Eye Irritation Test

OECD TG 405 is an internationally recognized standard in vivo test protocol for evaluating the potential for acute irritation and corrosion that chemical substances and mixtures may cause to eye tissues. The basic principle of the test involves applying a specific amount of the test substance (typically 0.1 mL of liquid or 100 mg of solid) to the conjunctival sac of the test subject’s eye in a single application and grading the ocular reactions at specific time points. The evaluation is based on four main parameters: corneal opacity, iris damage, conjunctival redness, and conjunctival edema. Each parameter is scored on a scale from 0 (none) to 4 (severe), and the time-dependent course of these scores is critical in determining whether the substance’s effect on the eye is reversible.

The most important feature of TG 405 is the stepwise testing strategy required prior to initiating the test. This strategy was developed to prevent unnecessary use of animals and to protect animal welfare to the highest degree. In the first stage, existing human and animal data, structure-activity relationship analyses, and physicochemical properties are evaluated. In particular, substances with a pH value of ≤2 or ≥11.5 are directly classified as “Eye Corrosive” (Category 1) due to their strong acid/base characteristics, and no in vivo testing is performed. In the second stage, the potential for corrosion and serious damage is screened in advance using validated in vitro and ex vivo tests (OECD TG 437, 438, 491, 492). However, when these steps are insufficient to definitively classify a substance’s potential for eye irritation, the in vivo OECD TG 405 test is performed as a last resort.

In our laboratory in vivo TG 405 testing is unavoidable, we begin the test with a single subject and, based on the results, include additional subjects only when necessary. This sequential test design ensures scientific validity while keeping the number of animals to a minimum. In addition, experienced veterinarians continuously monitor the animals throughout the test; in the event of a severe reaction, the test is immediately terminated, and appropriate analgesic treatment is administered.

Modern Alternatives: In Vitro and Ex Vivo Methods

Today, there are numerous in vitro and ex vivo methods—developed as alternatives to animal testing and validated by the OECD—for evaluating eye irritation. These methods are increasingly preferred not only for their compliance with the 3R principles but also because they provide faster, more economical, and more reproducible results.

OECD TG 437 (BCOP – Bovine Corneal Opacity and Permeability Test) is an ex vivo test that uses the corneas of bovine eyes obtained from slaughterhouses. After the test substance is applied to the cornea, corneal opacity (cloudiness) and permeability are measured. The in vitro irritation score, derived by combining these two parameters, is used to assess the substance’s potential for eye irritation. The BCOP provides high accuracy, particularly in the identification of severe irritants and corrosive substances.

OECD TG 438 (ICE – Isolated Chicken Eye Test) is an ex vivo test that uses chicken eyes. The test substance is applied to an isolated chicken eye, and changes in the cornea, iris, and conjunctiva are evaluated. The ICE test is particularly effective in assessing the irritation potential of substances such as surfactants and alcohols.

OECD TG 491 (STE – Short-Term In Vitro Exposure Test) is an in vitro test performed using a human corneal epithelial cell line. The test substance is applied to the cells for a short period, and cell viability is measured using the MTT assay. STE is used specifically to assess the irritation potential of water-soluble substances.

OECD TG 492 (RhCE – Reconstituted Human Corneal-Like Epithelial Test) is an in vitro test that uses three-dimensional tissue models mimicking human corneal epithelium. The test substance is applied to the tissue model, and cell viability is measured. RhCE is a preferred method, particularly for cosmetics and personal care products, because animal testing is prohibited under the EU Cosmetics Regulation.

The table below summarizes the key OECD guidelines used in eye irritation assessment and their characteristics.

GHS Classification and Eye Irritation Categories

Eye irritation test results are classified into three main categories according to the Globally Harmonized System (GHS) criteria. Category 1 (Serious Eye Damage / Eye Corrosive) is characterized by irreversible ocular tissue damage or permanent impairment of the cornea, iris, or conjunctiva in at least one test subject within 21 days. This category is labeled with the statement “Causes serious eye damage” and marked with the corrosive pictogram (GHS05). Category 2A (Eye Irritant) is characterized by positive ocular reactions that are fully reversible within 21 days in at least two test subjects; it is labeled with the statement “Causes serious eye irritation” and marked with the exclamation mark pictogram (GHS07). Category 2B (Mild Eye Irritant), on the other hand, covers mild and transient effects that do not fully meet the Category 2A criteria but resolve completely within 7 days; this category is not used in some regulations.

This classification is directly used to determine numerous regulatory requirements, ranging from product labeling to transportation, storage, and instructions for use.
Incorrect or incomplete classification can lead to both legal penalties and risks to user safety. Therefore, the proper design and meticulous execution of eye irritation testing are critical factors that directly impact your product’s reliability and regulatory compliance in the market.

Conducting Eye Irritation Tests in Our Laboratory

In our GLP-certified laboratory, we conduct eye irritation and corrosion tests in full compliance with international standards. Our testing strategy follows the stepwise approach recommended by the OECD: first, an evaluation of existing data and physicochemical properties; then, the application of validated in vitro/ex vivo tests; and, as a last resort, the performance of in vivo testing. This strategy ensures the highest levels of both scientific validity and ethical responsibility.

For in vitro and ex vivo tests, our laboratory offers the BCOP (TG 437), ICE (TG 438), STE (TG 491), and RhCE (TG 492) methods in their entirety. These tests significantly reduce the use of animals while allowing for more direct extrapolation to the human eye. Particularly for cosmetics and personal care products, since animal testing is prohibited under the EU Cosmetics Regulation, these in vitro methods are the only mandatory option.

In cases where in vivo testing is unavoidable, we operate in full compliance with the 3R principles. We begin the test with a single subject and include additional subjects only when necessary, based on the results. Experienced veterinarians monitor the subjects continuously throughout the test; in the event of a severe reaction, the test is immediately terminated, and appropriate analgesic treatment is administered. All observations are recorded in accordance with GLP-compliant standard operating procedures.

One of our laboratory’s key advantages is that test results can be directly integrated into your regulatory submissions. Comprehensive study reports compliant with the OECD template meet the requirements of the EU REACH and CLP Regulations, the U.S. EPA’s FIFRA and TSCA, and Turkey’s KKDIK. In addition, we provide all the necessary data in full for preparing your product’s Safety Data Sheet (SDS).

Frequently Asked Questions

One of the most common questions we receive regarding the eye irritation test is which OECD guideline is appropriate for which product. The selection depends on the product’s physical form, solubility, and your regulatory objective. For water-soluble substances, TG 491 (STE) may be appropriate; for water-insoluble substances, TG 437 (BCOP) or TG 492 (RhCE); and for surfactants, TG 438 (ICE). When in vivo testing is required for definitive classification, TG 405 is used. Our laboratory evaluates your product’s characteristics to recommend the most appropriate method.

Another common question is whether TG 405 testing is required for a substance with an extreme pH value.
No. Substances with a pH ≤2 or ≥11.5 are directly classified as “Serious Eye Damage / Corrosive” (Category 1) without applying OECD TG 405. This approach has been mandated by the OECD to prevent unnecessary animal use.

It is also frequently asked whether in vitro test results can serve as a substitute for OECD TG 405. Yes, in many cases they can. In particular, validated in vitro tests, such as BCOP and RhCE, may be sufficient for classifying a substance as “Category 1” (corrosive/serious eye damage) or “non-Category 2” (non-irritating). However, for the Category 2A/2B distinction, an in vivo test may be required in some cases. Regarding the test duration, the in vivo TG 405 test takes approximately 3–4 weeks, including an observation period that can last up to 21 days; in vitro alternative tests, on the other hand, typically yield results within 1–2 weeks. Including reporting and quality assurance reviews, the total duration is 4–6 weeks.

The eyes are our most vulnerable organs against chemical hazards, and scientifically demonstrating this sensitivity should be a priority for every responsible manufacturer. Using OECD TG 405 and complementary in vitro alternative tests, our laboratory establishes the ocular safety profile of your products in accordance with the highest scientific standards and in full compliance with the 3R principles. For your eye irritation and corrosion assessment needs, contact our GLP-certified laboratory, and let’s plan your customized testing strategy together.