Diazolidinyl Urea: A Comprehensive Reference
1. Identity and Chemical Profile
Names and Nomenclature
Diazolidinyl urea is an antimicrobial preservative used in cosmetics. Its IUPAC chemical name is 1-[1,3-Bis(Hydroxymethyl)-2,5-Dioxoimidazolidin-4-Yl]-1,3-Bis(Hydroxymethyl)Urea. Its CAS number is 78491-02-8 and its EC number is 278-928-2. It is also known as Diazolidinylurea or Germall II. The INCI (International Nomenclature of Cosmetic Ingredients) name used on product labels is simply DIAZOLIDINYL UREA. Other synonyms and trade names that appear in technical and commercial literature include Germaben II (when blended with other preservatives), Microcide IV, NeoCide GML-2, and Germall Diazolidinyl Urea. Its COSING REF No. is 33218 and its EINECS/ELINCS number is 278-928-2.
Molecular Formula and Physical Properties
The chemical formula of diazolidinyl urea is C₈H₁₄N₄O₇ and its molecular weight is 270.22 g/mol. It is a colorless, odorless, stable, and water-soluble preservative. In its raw form, diazolidinyl urea appears as a fine white powder. It is soluble in water and alcohol and is effective over a wide pH range (3–9).
Structural Complexity and Commercial Form
Commercial diazolidinyl urea is a mixture of different formaldehyde addition products including polymers. It is produced by the chemical reaction of allantoin and formaldehyde in the presence of sodium hydroxide solution and heat. The reaction mixture is then neutralized with hydrochloric acid and evaporated. Diazolidinyl urea was poorly characterized until recently, and the single Chemical Abstracts Service structure assigned to it is probably not the major one in the commercial material. New data show that the hydroxymethyl functional group of the imidazolidine ring is attached to the carbon, not the nitrogen atom.
Diazolidinyl urea is a heterocyclic-substituted urea that functions as a preservative in cosmetics and personal care products. It is manufactured by the reaction of allantoin and formaldehyde. Allantoin, a compound found in plants and animals, undergoes a chemical reaction with formaldehyde under alkaline conditions to produce diazolidinyl urea. Allantoin itself is a naturally occurring compound found in the comfrey plant (Symphytum officinale) and in various other organisms; however, diazolidinyl urea itself is exclusively a synthetic product of controlled industrial chemistry and has no direct natural source or analogue.
Classification
Diazolidinyl urea is classified as a regulated, formaldehyde-releasing preservative. It is regulated in Europe, Brazil, the United Kingdom, and Washington State (USA) under the Toxic-Free Cosmetics Act (TFCA). It is chemically related to imidazolidinyl urea, which is used in the same way. Diazolidinyl urea acts as a formaldehyde releaser.
2. Historical Introduction and Use
Diazolidinyl urea was first introduced in 1982 by Sutton Labs, Chatham, New Jersey. It was developed as a second-generation improvement upon imidazolidinyl urea (Germall 115), with a broader antimicrobial spectrum. Diazolidinyl urea has a wider antimicrobial spectrum than imidazolidinyl urea, to which it is structurally related. The compound has no traditional or pre-modern history of use in any culture or herbal medicine system. It is an entirely synthetic molecule invented in the era of modern cosmetic chemistry. Its application has been exclusively industrial — specifically for the preservation of personal care and cosmetic product formulations. There are no records of traditional, ethnobotanical, Ayurvedic, traditional Chinese medicine, or other historical medicinal applications for diazolidinyl urea.
It has been used in many cosmetics, skin care products, shampoos and conditioners, as well as a wide range of products including bubble baths, baby wipes, and household detergents. Diazolidinyl urea is found in the commercially available preservative Germaben. The Germaben II formulation by Ashland Specialty Chemical — containing propylene glycol, diazolidinyl urea, methylparaben, and propylparaben — became one of the most widely used commercial preservative blends across multiple product categories in the cosmetics industry from the 1980s onward.
3. Key Constituents and Chemistry
Decomposition Products
Diazolidinyl urea is decomposed to (4-hydroxymethyl-2,5-dioxo-imidazolidine-4-yl)-urea (HU) and (3,4-bis-hydroxymethyl-2,5-dioxo-imidazolidine-4-yl)-urea (3,4-BHU) in most cosmetic samples tested. In aqueous solution, diazolidinyl urea is decomposed to release formaldehyde. One molecule of diazolidinyl urea can release 4 molecules of formaldehyde under rigorous conditions.
After five minutes of reaction time, diazolidinyl urea released a total of 2.1 moles of formaldehyde per mole of preservative as determined by the Hantzsch reaction. This differed from the predicted value of 4.0 moles, but the value of 2.0 moles was used for all further calculations.
Degradation products shared by both diazolidinyl urea and imidazolidinyl urea include HU and 3,4-BHU, which are among the molecules responsible for cosensitization. Approximately 11% and 63% of patients who react to imidazolidinyl urea also show reactivity to formaldehyde. When imidazolidinyl urea or diazolidinyl urea show cross-reactivity to quaternium-15, the responsible molecule is considered to be formaldehyde, as the agents are not structurally related. Manufacturers use formaldehyde-releasing preservatives, which include Quaternium-15, imidazolidinyl urea, diazolidinyl urea, and DMDM hydantoin, substances that still top the list of allergens causing positive patch-test reactions.
Compatibility with Formulation Ingredients
Diazolidinyl urea is compatible with most cosmetic ingredients. It is not inactivated by anionic, cationic, or nonionic surfactants or proteins. This property makes it particularly useful in complex multicomponent formulations. However, the presence of proteins and other reactive biomolecules in formulations can influence the net amount of free formaldehyde measurable, since these species can bind with and partially neutralize the released formaldehyde.
4. Mechanism of Action
Antimicrobial Mechanism
Functioning as a formaldehyde releaser, diazolidinyl urea acts by slowly releasing formaldehyde when in contact with water, thereby inhibiting microbial growth in various formulations. Its mechanism involves the reaction of formaldehyde with amino and hydroxyl groups of proteins and nucleic acids, ultimately disrupting microbial cell structures and functions.
It is reported to have a wider antimicrobial spectrum than imidazolidinyl urea, which is structurally related. It is effective against gram-negative and gram-positive bacteria, molds, and yeast but has limited activity against fungi. Diazolidinyl urea is effective against most contaminating microorganisms, especially Pseudomonas species.
Formaldehyde-releasing preservatives are chemicals used in some skincare and cosmetics to prevent the growth of bacteria and extend their shelf life. These preservatives work by gradually releasing small amounts of formaldehyde over time. The slow, sustained release of formaldehyde is considered advantageous from a formulation standpoint, because the low, sub-threshold concentration of the released aldehyde exerts antimicrobial activity without the aggressive tissue reactivity of directly added free formaldehyde.
5. Formulations, Product Types, and Usage Levels
Product Applications
Diazolidinyl urea is a fine white powder. It can be found in many cosmetic and personal care product types including eye and facial makeup, aftershave, and nail, bath, hair, and skin care products. Its antimicrobial properties inhibit the growth of bacteria, yeast, and mold, extending the shelf life of various formulations, including creams, lotions, shampoos, and makeup.
Germaben II, a commercial blend containing diazolidinyl urea, finds application in formulating leave-on and rinse-off formulas, hair-care products (conditioners, gels, mousses, stylers, shampoos, styling lotions/creams), body-care, color cosmetics, face and body washes, facial care and wipes, and sun-care products (after-sun, self-tanning, and sun-protection).
Usage Concentrations Reported in Studies and Regulatory Guidance
Diazolidinyl urea is a heterocyclic-substituted urea used as a preservative in a variety of cosmetic products at a normal product use concentration of 0.2 to 0.4%, up to a maximum of 1.0%. Generally, it is used in concentrations of 0.03 to 0.3 percent. Usage level is typically 0.1–0.3% in cosmetic formulations.
At concentrations up to 0.4%, diazolidinyl urea was a mild cumulative skin irritant in humans. It was not a sensitizer in a repeat insult patch test on nonpatient volunteers. Diazolidinyl urea was not a photosensitizer at 0.25%.
Diazolidinyl urea was added to anionic shampoos, both with and without protein, at concentrations of 0.1, 0.2, 0.4, and 0.8%, and the amount of formaldehyde released was measured at both 23°C and 60°C. These measurement studies helped inform the regulatory limit-setting process.
The genotoxicity study by Pfuhler and Wolf (2002) used diazolidinyl urea in the range of 0.04–1.8 μmol per plate in the Salmonella typhimurium mutagenicity assay. The in vitro neural cell study used 1–50 μM diazolidinyl urea or imidazolidinyl urea at various time points to measure reactive oxygen species (ROS) in rat neural progenitor cells.
6. Scientific Evidence by Area of Use
6.1 Antimicrobial Preservation (Primary Function)
The principal application of diazolidinyl urea is as an antimicrobial preservative in cosmetics and personal care products. The evidence for efficacy in this role is well-established through decades of industrial use, regulatory review, and laboratory testing.
Diazolidinyl urea is an antiseptic and deodorizer. It is also a broad-spectrum preservative against bacteria and fungi. Research has focused on optimizing diazolidinyl urea concentrations in cosmetic products to ensure effective antimicrobial activity while minimizing potential adverse effects, such as skin irritation or sensitization. Additionally, investigations have explored its compatibility with different formulation ingredients and its stability under various storage conditions.
Evidence strength: The antimicrobial efficacy of diazolidinyl urea is well-established through formulation challenge testing and regulatory review. There are no published randomized controlled trials (RCTs) examining its antimicrobial action in humans, as this is not a directly consumer-facing therapeutic claim — rather, it functions as an ingredient-level preservative. The regulatory and toxicological literature treats its preservative efficacy as scientifically settled.
6.2 Contact Allergy and Skin Sensitization
This is the area with the most extensive clinical and human evidence. Contact allergy to diazolidinyl urea has been documented in numerous patch test studies and case series across multiple countries.
Four cases of contact allergy to diazolidinyl urea (Germall II) in a "hypoallergenic" brand of cosmetics were described as early as the late 1980s. Two patients sensitized by these cosmetics were not allergic to formaldehyde. These cases established a key point: sensitization to diazolidinyl urea can occur independently of formaldehyde sensitivity.
Contact allergy to diazolidinyl urea may or may not be due to formaldehyde sensitivity. Patients allergic to formaldehyde may suffer allergic reactions from cosmetics containing diazolidinyl urea. Patients sensitized to diazolidinyl urea may cross-react to imidazolidinyl urea and vice versa. It has been suggested that the sensitizing potential of diazolidinyl urea is greater than that of imidazolidinyl urea.
A clinical study reviewing 708 consecutive patch-tested patients investigated whether diazolidinyl urea sensitization is secondary to formaldehyde release or due to its own allergenic properties. Profiles of the 58 individuals (8%) with diazolidinyl urea sensitivity were analyzed. Significant coexistent biocide reactivity was demonstrated for diazolidinyl urea and formaldehyde (81%); 12% reacted to diazolidinyl urea alone. The primary mode of sensitization was concluded to be via formaldehyde release, while independent contact allergy was less frequent.
In a Finnish study of patch-test reactions to cosmetic allergens, diazolidinyl urea was responsible for only 0.9% of allergic reactions in 6,539 patients.
In a clinical trial examining patch test standardization, the preservatives imidazolidinyl urea (IMID) and diazolidinyl urea (DU) were described as commonly used in cosmetic products and well-known sensitizers. The study included 181 patients, of whom 150 were referred for patch testing, 12 were patients with known allergy to IMID, and 19 were patients with known allergy to DU.
Two leg ulcer patients were found to be sensitized to different commercial hydrogels, with concomitant patch test positivity to imidazolidinyl urea and diazolidinyl urea. Patient A was an 84-year-old male and patient B was a 77-year-old female, both with a known history of leg ulcer for more than 8 weeks. This case series highlights the relevance of these preservatives in medical-grade topical products, not only cosmetics.
Formaldehyde-releasing preservatives, such as quaternium-15, diazolidinyl urea, and imidazolidinyl urea, are widely used in cosmetics and topical medications and are well-known contact sensitizers. Despite positive patch test reactions in a number of patients, only some of these patients will react when they use the corresponding commercial formulations. This is because the concentrations of preservatives in the commercial products are often below the threshold necessary to produce a clinical reaction. This finding confirms the importance of using commercial formulations in estimating the clinical relevance of patch test results.
Some preservatives, such as parabens, diazolidinyl urea, and BHT (butylhydroxytoluene), are mainly used in topical medications and are considered the second most common group of allergens in cosmetics after fragrances, with prevalence rates of up to 32%.
Evidence strength: Strong and well-replicated human evidence. Multiple cross-sectional studies, case series, and retrospective analyses have documented contact allergy to diazolidinyl urea across diverse patient populations. The mechanism is understood as involving both formaldehyde-mediated and possibly direct sensitization.
6.3 Genotoxicity and Mutagenicity
Several laboratory studies have examined whether diazolidinyl urea or its released formaldehyde poses genotoxic risk.
Diazolidinyl urea (DZU) was tested for genotoxicity in three short-term test systems: the Salmonella typhimurium mutagenicity assay, the in vitro micronucleus test with V79 Chinese hamster cells, and the in vitro tubulin assembly assay using isolated tubulin from pig brains. In the Salmonella typhimurium mutagenicity assay, diazolidinyl urea showed a clear and concentration-dependent increase in the number of revertants in strains TA98, TA100, and TA102, with and without metabolic activation (rat liver S9 mix). A biologically relevant increase in the number of revertants was achieved within the concentration range tested (DZU: 0.04–1.8 μmol per plate). The authors attributed these in vitro genotoxic signals primarily to the formaldehyde released by the compound.
In contrast, the Cosmetic Ingredient Review's earlier assessment reported more equivocal mutagenicity findings: diazolidinyl urea was nonmutagenic when tested in bacteria or in the micronucleus assay. Diazolidinyl urea was found non-mutagenic in two Ames tests and a micronucleus assay in vivo in CD1 mice (CIR, 1990). These discrepancies across studies may reflect differences in test protocols and the specific formaldehyde content of the test batches used.
Female Sprague-Dawley rats exposed to 3,140 mg/kg/day of a cosmetic product containing 0.2% of diazolidinyl urea (~6 mg/kg) for 13 weeks did not exhibit any systemic toxicity. Genotoxicity concerns arise from the presence of formaldehyde in products containing these preservatives. While formaldehyde is classified as hazardous (Category 2 carcinogenic substance) with the risk phrase "May cause cancer by inhalation," this applies to inhaled formaldehyde at high concentrations. Formaldehyde is not likely to be volatile from the low concentration solutions present in products containing these preservatives.
Evidence strength: Preliminary and mixed. In vitro genotoxic signals have been observed at concentrations substantially above those found in typical cosmetic use. In vivo animal studies have not confirmed systemic genotoxic risk at use-relevant doses. No human epidemiological data link cosmetic use of diazolidinyl urea to genotoxic outcomes.
6.4 Neurotoxicity (In Vitro Only)
A 2018 in vitro study examined the potential neurotoxicity of diazolidinyl urea and related cosmetic preservatives. The study investigated the effects of benzalkonium chloride, diazolidinyl urea, and imidazolidinyl urea on ROS-dependent apoptosis of rat neural progenitor cells in vitro. These are commonly used preservatives in cosmetics. Recent reports suggested that these compounds may have cellular and systemic toxicity at high concentrations. Diazolidinyl urea and imidazolidinyl urea are known formaldehyde releasers, raising concerns for these cosmetic preservatives.
Cultured cells were treated with 1–50 μM diazolidinyl urea or imidazolidinyl urea at various time points to measure reactive oxygen species (ROS). PI staining, MTT assay, and live-cell imaging were used for cell viability measurements. In rat NPCs, ROS production and cleaved caspase-8 expression were increased while cell viability was decreased at high concentrations. These results suggest that several cosmetic preservatives at high concentrations can induce neural toxicity in rat brains through ROS induction and apoptosis.
The CIR Expert Panel has stated that up to 0.5% diazolidinyl urea is safe, but the assurance of safety from repeated exposures still needs to be determined.
Evidence strength: Preliminary and limited. All findings derive from in vitro cell culture using rat embryonic neural cells exposed to concentrations that may not reflect realistic human dermal exposure. No human or in vivo animal data exist on neurotoxic outcomes from topically applied diazolidinyl urea at cosmetic use concentrations.
7. Body Systems and Health Areas Associated with Diazolidinyl Urea
- Skin / Integumentary system: The primary area of relevance. Diazolidinyl urea is topically applied to the skin via cosmetic formulations. Clinical evidence documents its role as a contact allergen and mild skin irritant. Some people can be sensitive or allergic to formaldehyde-releasing preservatives. These ingredients can potentially cause skin irritation and allergic reactions, such as dermatitis.
- Immune system / allergic responses: Diazolidinyl urea has been found to be a stronger sensitizer than imidazolidinyl urea for people sensitive or allergic to formaldehyde. It is classified as a contact allergen and is included in standard patch test series for the investigation of allergic contact dermatitis.
- Central nervous system (in vitro only): Addressed above. Findings are confined to isolated cell culture experiments and have not been reproduced in vivo or in human populations.
- Ocular mucosa: The Cosmetic Ingredient Review Expert Panel evaluated the safety of diazolidinyl urea and found that the ingredient has low eye irritation potential. At 5%, diazolidinyl urea was not an ocular or skin irritant.
- Systemic toxicity: The CIR Expert Panel noted that diazolidinyl urea was relatively non-toxic in oral subchronic studies.
8. Regulatory Status
United States (CIR / FDA)
The CIR Expert Panel evaluated the scientific data and concluded that diazolidinyl urea was safe as a cosmetic ingredient up to a maximum concentration of 0.5%. In 2006, as part of the scheduled re-review of ingredients, the CIR Expert Panel reconsidered available new data on this ingredient and reaffirmed the above conclusion. The CIR is an independent review body whose panel includes dermatologists, toxicologists, pharmacologists, and veterinary medicine specialists, with participation by the U.S. Food and Drug Administration (FDA) and the Consumer Federation of America.
The CIR Expert Panel concluded that there was no indication that the use of diazolidinyl urea in cosmetics and personal care products would release formaldehyde at concentrations which would exceed their previously recommended limits for formaldehyde (0.2%).
In three studies of sensitization potential, diazolidinyl urea was a mild sensitizer, but was not a sensitizer in a fourth study.
European Union
The EU working party identified diazolidinyl urea as one of four formaldehyde releasers permitted to be used as preservatives in cosmetic products according to Directive 76/768/EC. The European Union's Cosmetic Regulation also authorizes its use up to a maximum concentration of 0.5%. Diazolidinyl urea may be used in cosmetics and personal care products at a maximum authorized concentration of 0.5% and must be labeled "contains formaldehyde" if the concentration of formaldehyde in the finished product exceeds 0.05%.
The same rules that apply to free formaldehyde also apply to formaldehyde-releasing preservatives such as DMDM hydantoin, imidazolidinyl urea, diazolidinyl urea, quaternium-15, bronopol, and sodium hydroxymethylglycinate. These ingredients do not contain free formaldehyde initially but release it over time to provide antimicrobial benefits. The EU treats the formaldehyde they release with the same caution as free formaldehyde added directly to products.
Notably, the EU regulatory landscape for formaldehyde releasers including diazolidinyl urea underwent significant tightening beginning in 2022. In its scientific advice (SCCS/1632/21), the SCCS concluded that the previous threshold could not adequately protect consumers sensitized to formaldehyde, and recommended lowering the labelling threshold to 0.001% (10 ppm) for total formaldehyde released, regardless of whether the product contains one or more formaldehyde releasers. In light of this advice, the EU introduced Commission Regulation (EU) 2022/1181 in July 2022, amending the Cosmetics Regulation to lower the labelling threshold for formaldehyde releasers listed in Annex V.
From 15 July 2026, a new mandatory labelling requirement for formaldehyde-releasing preservatives comes into force: a warning is required when released formaldehyde exceeds 0.001% (10 ppm). The SCCS concluded that the previous 0.05% threshold was insufficient to protect consumers already sensitized to formaldehyde from developing contact dermatitis.
9. Safety Considerations
Contact Allergy and Sensitization
Diazolidinyl urea has been found to be a stronger sensitizer than imidazolidinyl urea for people sensitive or allergic to formaldehyde. Clinical patch test data across multiple studies consistently identify it as one of the more allergenic cosmetic preservatives available, particularly in populations with pre-existing formaldehyde sensitivity. The concentrations of preservatives in commercial products are often below the threshold necessary to produce a clinical reaction, which means that positive patch tests do not always translate to clinically relevant reactions during actual product use.
The diazolidinyl urea-derived decomposition products differ between cosmetics and patch test preparations. To test the contact sensitivity of the diazolidinyl urea present in cosmetics, patch tests with HU and 3,4-BHU in petrolatum should be performed. This finding has practical implications for the diagnostic accuracy of standard patch testing protocols.
Cross-Reactivity
Cross-reactivity to diazolidinyl urea is common among patients sensitized to imidazolidinyl urea, and vice versa. The two former urea constituents (HU and 3,4-BHU) are also common in diazolidinyl urea and may be possible causative agents in cross-reactivity.
Formaldehyde Release and Carcinogenicity Context
Formaldehyde is classified as a CMR 1B substance (carcinogenic, mutagenic, or toxic for reproduction) under the EU's CLP Regulation, warranting close regulatory oversight. This classification applies to formaldehyde itself, primarily as an inhaled substance. While formaldehyde is classified as hazardous with the risk phrase "May cause cancer by inhalation," this applies to inhaled formaldehyde at high concentrations. Formaldehyde is not likely to be volatile from the low concentration solutions present in products containing these preservatives.
The CIR report noted that diazolidinyl urea is a formaldehyde releaser. The CIR Expert Panel previously concluded that the use of formaldehyde in cosmetics and personal care products was safe to the great majority of consumers. The CIR Expert Panel concluded that there was no indication that the use of diazolidinyl urea in cosmetics and personal care products would release formaldehyde at concentrations which would exceed their recommended limits for formaldehyde (0.2%).
Skin Irritation
A study from 1990 found that at concentrations up to 0.4%, diazolidinyl urea was a mild cumulative skin irritant. The CIR reviewed it in 2006 and found that, at concentrations below 0.5%, it is safe as used, as the amount of formaldehyde released will be smaller than the recommended limit of less than 0.2%.
Diazolidinyl urea has considerable risks in the form of skin sensitivities like irritation and redness, especially when used at concentrations higher than 0.5%. Due to its formaldehyde-releasing nature, it can exacerbate symptoms in those sensitive to formaldehyde. Individuals with existing skin conditions or compromised skin barriers may be more susceptible to these side effects.
Photosensitization
Diazolidinyl urea was not a photosensitizer at 0.25%. No published data indicate phototoxic effects at concentrations used in cosmetics.
Occupational Exposure
Diazolidinyl urea, a formaldehyde releaser, is contained mainly in cosmetics and toiletries and can be found in barrier creams. Workers in cosmetic manufacturing or professional hairdressing environments with repeated, high-frequency exposure may be at elevated risk for occupational contact sensitization compared with typical consumer use.
Interactions with Other Preservatives
Diazolidinyl urea is frequently used in combination with other preservatives. The commercially popular Germaben II formulation contains propylene glycol, diazolidinyl urea, methylparaben, and propylparaben. The combination of diazolidinyl urea with parabens broadens the spectrum of antimicrobial activity. Patients sensitized to diazolidinyl urea who also use paraben-containing products may require patch testing of the full preservative system to identify all relevant allergens.
10. Limitations and Evidence Gaps
Several significant evidence gaps exist in the scientific literature on diazolidinyl urea:
- There are no published prospective randomized controlled trials examining human skin sensitization rates under controlled cosmetic product use conditions.
- Long-term epidemiological data on cumulative low-dose formaldehyde exposure from cosmetic preservatives — including diazolidinyl urea — in relation to chronic disease outcomes (e.g., cancer) are absent.
- In vitro findings (genotoxicity, neurotoxicity) are not replicated in robust in vivo or human studies at realistic exposure concentrations.
- The diazolidinyl urea-derived decomposition products differ between cosmetics and patch test preparations, meaning that clinical patch testing with the neat compound may not perfectly reflect sensitization risk from formulated products in real-world use.
- The assurance of safety from repeated exposures to diazolidinyl urea still needs to be determined.
References
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