Solid Shampoo Is Mostly Not Preserved, in Contrast to Liquid Shampoos Used in Hairdressing—A Market Survey in Four European Countries

ABSTRACT

Background

Hairdressers are frequently exposed to shampoos, both from neat liquid products and dilutions, often with unprotected hands. Shampoos usually contain preservatives. While the level of preservation is guided by risk assessment based on typical consumer exposure, this will underestimate sensitization risk in hairdressers. Lower exposure or avoidance of preservative‐containing hair cosmetic products appears important for primary prevention of work‐related allergic contact dermatitis among hairdressers.

Objectives

To assess the preservatives (Annex V of the EU Cosmetics Regulation (EC 1223/2009)) used in solid shampoos, compare these to preservatives present in liquid shampoos for professional use, and determine the presence of ingredients with “antimicrobial” function in solid shampoo.

Methods

A physical store check and survey of online sales platforms was performed in four European countries in late 2024. Along with product descriptors, the full INCI labelling was recorded from package labels (or corresponding summaries online) and collected in a spreadsheet.

Results

The sample included 172 solid shampoos from various retail sources and 259 liquid shampoos from one online marketplace for professional users. Among solid shampoos, preservatives were found in 31.4%; if “antimicrobial” additives are additionally considered, this share increased to 41.3%. Benzyl alcohol (n = 29) and sodium benzoate (n = 18) were the most common preservatives, benzyl benzoate the most common “antimicrobial” (n = 8). By contrast, all liquid shampoos contained (multiple) preservatives.

Conclusions

Solid shampoo shows potential to provide a less‐allergenic working environment for hairdressers. Practical aspects should next be addressed to contribute to broader use of solid shampoos.

Most solid shampoos (68.6%) did not contain preservatives according to Annex V of the EU Cosmetics Regulation (EC 1223/2009), while all liquid shampoos did. The number of preservatives used was significantly greater in liquid shampoos.

1. Introduction

Most hairdressers perform hair washes using liquid shampoos mixed with water on multiple clients per day, often without wearing protective gloves. According to a recent review, this will result in more skin exposure on the hands compared to consumers, estimated to be 6–13 times higher assuming no glove use at all [1]. Such exaggerated cumulative exposure is regarded a likely explanation for a higher risk of contact allergy to preservatives such as methylisothiazolinone (MI; prevalence ratio [PR] 3.12, 95% confidence interval [CI] 2.4–4.0) and (historically) methyldibromo glutaronitrile (MDBGN; PR 2.4, 95% CI 1.9–3.1) found in a long‐term analysis of patch test data in female hairdressers compared to female clients with hair cosmetic‐related contact dermatitis [2]. In absolute terms, the most recent detailed analysis of the above demonstrated an age‐standardised prevalence of MI sensitization of 10.5% in hairdressers versus 3.1% in consumers [3]. Use of solid shampoo in lieu of liquid shampoos could be considered to reduce skin exposure to preservatives in hairdressers. Before such a recommendation, however, the actual presence of preservatives in solid shampoos on the market needs to be assessed and compared with liquid shampoos used commercially, which was the main objective of our study.

2. Methods

Labelled ingredients of solid shampoos found in several regular consumer retail outlets (grocery stores, pharmacies) as well as online stores were checked in Denmark (DK), Germany (DE), Switzerland (CH), and the United Kingdom (UK) during the second half of 2024. Additionally, hair salons were visited in Denmark to ask whether they sell solid shampoo. Product and labelling information were photographed on‐site or copied from online content, respectively, and the information later transferred into a spreadsheet. This convenience sampling method was not representative in a formal sense similar to previous studies capturing data on products available on the consumer and/or professional market. To ease data entry, an initial spreadsheet with the 105 columns (i.e., different ingredients encountered and recorded) in Germany was made available to the other researchers. However, many new ingredients were later identified and added at all three other sites, finally arriving at 420 different ingredients after alignment of duplicate ingredients. In case of natural substances (“botanicals”), a new ingredient name was added to represent possible differences in raw material preparation of the same plant species resulting in potential differences in allergenic potential. However, such nomenclative uncertainties were not encountered with regard to preservatives which are the focus of the present study.

The online market platform of a German supplier of professional hair cosmetics (https://www.friseur‐einkauf.com, last accessed 2025‐04‐22) was used to obtain information on solid shampoos as well as liquid, water‐based shampoos for comparison. Information on ingredients was retrieved from this website or, if rarely unavailable, from the manufacturer's website. While “2 in 1” and “hair and body” shampoos were included, conditioners and dry shampoos were not. For liquid shampoo, data on preservatives only were collected from the ingredients as this is the focus of this study. Preservatives considered in this study included ingredients listed in Annex V of the EU Cosmetics Regulation (EC 1223/2009). In addition, the use of ingredients with “antimicrobial” function was included in the descriptive analysis for solid shampoos.

Differences in proportions between two disjunct subgroups were statistically tested with Fisher's exact test, using a Monte‐Carlo simulation of the exact p‐value for contingency tables larger than 2 × 2, using 2000 iterations and a random number as seed. Crude odds ratios (ORs) were calculated, accompanied by exact 95% confidence intervals (CIs). The R statistical program package (version 4.4.×, https://www.r‐project.org/) was used for analysis.

3. Results

3.1. Solid Shampoos

The store check identified n = 185 products in the four countries (DK: 38, DE: 43, CH: 44, and UK: 60). Duplicate products were removed, arriving at a final sample size of n = 172 solid shampoos. At the German online marketplace for hairdressers' professional supply, only five solid shampoos from two different companies were available, contrasting with a vast number of liquid shampoos.

The presence of preservatives in solid shampoo products marketed for consumers is shown in Table 1, including a mutual cross‐tabulation addressing co‐occurrence. The total number of occurrences is indicated in the rightmost column, benzyl alcohol and sodium benzoate being the most commonly encountered ingredients with 29 and 18 occurrences, respectively, in the total sample of 172 products. Eleven different preservatives were identified in the product sample. As some products contained more than one preservative, the total number of products preserved with any of the 11 preservatives encountered is lower than the sum of total occurrence of preservatives. Overall, 68.6% (118 of 172) of the solid shampoos did not contain preservatives in a strict sense.

TABLE 1.

Univariate occurrence (rightmost column) and co‐occurrence of those preservatives from Annex V of the EU Cosmetics Regulation (EC 1223/2009) found in a sample of solid shampoos in a market survey in four European countries performed during the second half of 2024.

	CAS Nr.	Na‐B	B‐acid	Me‐B	Ka‐So	So‐acid	P‐olam	PE	Dehy‐ac‐acid	Beh‐Cl	Cet‐Cl	Row total	p
Benzyl alcohol (BA)	100‐51‐6	1	3	2	1	0	2	2	5	1	0	29 (16.9%)	0.2
Sodium benzoate (Na‐B)	532‐32‐1		0	0	11	2	0	0	0	0	0	18 (10.5%)	< 0.0001
Benzoic acid (B‐acid)	65‐85‐0			2	0	0	0	1	1	0	0	3 (1.7%)	0.0001
Methyl benzoate (Me‐B) a	93‐58‐3				0	0	0	0	0	0	0	2 (1.2%)	—
Potassium sorbate (Ka‐So)	24634‐61‐5/590‐00‐1					0	0	0	0	0	0	14 (8.1%)	< 0.0001
Sorbic acid (So‐acid)	110‐44‐1						0	0	0	0	0	2 (1.2%)	0.06
Piroctone olamine (P‐olam)	68890‐66‐4							0	0	0	0	2 (1.2%)	0.2
Phenoxyethanol (PE)	122‐99‐6								1	0	0	4 (2.3%)	< 0.0001
Dehydroacetic acid (dehy‐ac‐acid)	520‐45‐6/771‐03‐9/16807‐48‐0									0	0	5 (2.9%)	0.04
Behentrimonium chloride (Beh‐Cl)	17301‐53‐0										3	4 (2.3%)	1.0
Cetrimonium chloride (Cet‐Cl)	112‐02‐7											3 (1.7%)	0.005

Note: “row total” percentages refer to the total sample of N = 172 as 100%. p‐Value refers to the result of a Fisher test comparing solid and liquid shampoo (in the latter sample, %ages were always higher, except for dehydroacetic acid, as a certain group presented in Table 4).

^a Maximum use concentration 0.5% acid, indicated in the Annex V entry number 1a. For other substances, see Table 4 for information.

However, other ingredients with alleged “antimicrobial” activity and partly other functions out of those 943 ingredients with this function listed in the CosIng database as of 24 June 2025, have been found to be present in solid shampoos, including many botanical extracts (Table 2). Both 2‐phenoxyethanol and cetrimonium chloride are “antimicrobial” ingredients and at the same time preservatives according to Annex V of the EU Cosmetics Regulation (EC 1223/2009); their occurrence is covered in Table 1. Altogether 26 products contained, usually one, antimicrobial, that is, 15.1% of all products. Co‐occurrence of preservatives and “antimicrobials” was seen in 9 solid shampoos. Thus, 58.7% (101/172) solid shampoos contained neither preservatives nor “antimicrobials”.

TABLE 2.

Occurrence of “antimicrobial” (poly)functional ingredients found in a sample of solid bar shampoos in a market survey in four European countries.

INCI ingredient name	CAS‐Nr.	n (%)
Benzyl benzoate	120‐51‐4	8 (4.7%)
Alcohol	64‐17‐5	7 (4.1%)
Zingiber officinale rhizome extract	84696‐15‐1	5 (2.9%)
Sodium caproyl/lauroyl lactylate	13557‐74‐9, 13557‐75‐0	2 (1.2%)
Citrus Grandis Peel Extract		2 (1.2%)
Chamomilla recutita (matricaria) flower extract	84082‐60‐0	1 (0.6%)
Cinnamomum zeylanicum bark extract	84649‐98‐9	1 (0.6%)
Humulus lupulus extract	8060‐28‐4, 8016‐25‐9	1 (0.6%)
Lawsonia inermis extract (henna)	84988‐66‐9	1 (0.6%)
Salvia officinalis leaf extract	8022‐56‐8, 84082‐79‐1	1 (0.6%)
Tagetes lemmonii leaf oil		1 (0.6%)

Note: Percentages refer to the total sample of N = 172 as 100%.

Use of preservatives (or “antimicrobials”) may be related to inclusion of water into the solid shampoo formulation, seen in 142 (82.6%) products according to qualitative ingredient labelling. There was a significant association between occurrence of water and preservatives in our sample (p = 0.005, Fisher's exact test), see Table 3A. If use of “antimicrobials” was additionally considered, the association between use of water and preservation/“antimicrobial” use remained significant (p = 0.01, OR = 3.4, 95% CI 1.2–10.7), as listed in Table 3B.

TABLE 3.

A: Co‐occurrence of water and preservatives according to Annex V of the EU Cosmetics Regulation (EC 1223/2009), and B: Water and preservatives or “antimicrobial” ingredients in a sample of N = 172 solid shampoos.

A		Preservatives		B	Preservatives/“antimicrobials”
		Yes	No		Yes	No	Row sum
Water	Yes	51	91		65	77	142
	No	3	27		6	24	30
Column total		54	118		71	101

To examine the relation, if any, between the amount of water and the presence and number of preservatives, respectively, the position of “aqua/eau/water” on the INCI ingredient labelling was determined. While 30 products did not contain water, water was ranked the first ingredient, that is, that with the largest share in product composition, in 4 solid shampoos, and found on the second rank in 17 products. The median rank of water was 3, the third quartile 6, and the lowest rank 21. If the rank of water is correlated with the number of preservatives used, the Spearman rank correlation coefficient is 0.211, that is, weak, but significant (p = 0.006).

If similar to above dichotomous cross‐tabulation use of “antimicrobials” or preservatives is considered, the Spearman rank correlation coefficient is 0.209, that is, similarly weak, and again significant (p = 0.006).

The five solid shampoos retrieved from the professional supply online marketplace all contained water, and all were at the third rank of ingredient labelling. The two products from one manufacturer (Davines) contained cetrimonium chloride, the three other products (KMS) all contained sodium benzoate, and one additionally contained potassium sorbate.

3.2. Liquid Shampoos

The sample included 259 liquid shampoos. All were preserved with one or several ingredients listed in Annex V of the EU Cosmetics Regulation (EC 1223/2009), which was a highly significant difference compared to solid shampoo (p < 0.00001, Fisher's exact test). Methylchloroisothiazolinone [MCI] and methylisothiazolinone [MI] were counted as one preservative if both were included. One of the Annex V preservatives was used in 30 products, two in 70 products, three in 48, four in 39, and five different preservatives in 40 liquid shampoos. In the remaining 32 products, 6 to up to 9 (in two cases) different preservatives were used. The number of preservatives used in liquid shampoo was significantly higher than in solid shampoo (p = 0.0005, Monte Carlo simulation of Fisher's exact test). The frequency of occurrence in the sample of single preservatives is shown in Table 4, rightmost column, along with additional regulatory information.

TABLE 4.

Preservatives according to Annex V of the EU Cosmetics Regulation (EC 1223/2009) identified in a sample of liquid (water‐based) shampoos marketed for professional use; maximum use concentrations indicated in the Annex V entry number “#.”

Ingredient	Annex V # (max. use conc.)	In liq. shampoo
Sodium Benzoate	# 1 (2.5%, acid)	204 (78.8%)
Phenoxyethanol	# 29 (1.0%)	131 (50.6%)
Salicylic Acid	# 3 (0.5%)	72 (27.8%)
Ethylparaben	# 12 (0.4% (as acid) for single ester a )	68 (26.3%)
Methylparaben	# 12 (0.4% (as acid) for single ester a )	68 (26.3%)
Potassium Sorbate	# 4 (0.6%, acid)	67 (25.9%)
Benzyl alcohol	# 34 (1.0%)	63 (24.3%)
Methylchloroisothiazolinone (MCI) with Methylisothiazolinone (MI)	# 39 (0.0015%)	56 (21.6%)
Methylisothiazolinone alone	# 57 (0.0015%), with MCI: see above	2 (0.8%)
Benzoic acid	# 1 (2.5%)	32 (12.4%)
Propylparaben	# 12 (0.4% (as acid) for single ester a )	28 (10.8%)
Cetrimonium Chloride	# 44 (0.1%)	22 (8.5%)
Sorbic acid	# 4 (0.6%)	13 (5.0%)
DMDM Hydantoin	# 33 (0.6%)	13 (5.0%)
Piroctone Olamine	# 35 (1.0%)	9 (3.5%)
Chlorphenesin	# 50 (0.3%)	6 (2.3%)
Iodopropynyl Butylcarbamate	# 56 (0.02%)	6 (2.3%)
Butylparaben	# 12a (0.14%) …	5 (1.9%)
Behentrimonium Chloride	# 44 (0.1%)	5 (1.9%)
Climbazole	# 32 (0.5%)	3 (1.2%)
Potassium Benzoate	# 1a (0.5% acid)	2 (0.8%)
Sodium Metabisulfite	# 9 (0.2% as free SO2)	2 (0.8%)
2‐bromo‐2‐nitropropane‐1,3‐diol	# 21 (0.1%)	1 (0.4%)
Diazolidinyl Urea	# 46 (0.5%)	1 (0.4%)
Chlorhexidine Digluconate	# 42 (0.3% as chlorhexidine)	1 (0.4%)
Dehydroacetic Acid	# 13 (0.6%)	1 (0.4%)
Polyaminopropyl Biguanide	# 28 (0.1%)	1 (0.4%)
Benzyl benzoate	Antimicrobial, Fragrance (III/85)	21 (8.1%)
p‐Cresol	Antimicrobial	4 (1.5%)
Isobutylparaben	Antimicrobial, II/1374 (prohibited)	3 (1.2%)
Glyoxal	Antimicrobial, III/194, max. 0.01% w/w	2 (0.8%)

Note: Rightmost column indicates number (%) of occurrence in the sample referring to the sample size of 259 as 100%. The last four rows include other ingredients with an “antimicrobial” function.

Abbreviations: liq, liquid, MCI, methylchloroisothiazolinone, MI, methylisothiazolinone.

^a 0.8% (as acid) for mixtures of esters.

4. Discussion

“Market survey” studies like the present one have proven useful in the past to achieve an overview on the diversity of ingredients in a certain group of cosmetic products. With some caution, they also provide a quantitative impression of their relative importance. Previous such studies have addressed mostly ingredients of (oxidative) hair dyes [4, 5, 6, 7, 8], but also natural ingredients [9].

4.1. Preservatives in Solid vs. Liquid Shampoo

4.1.1.

Liquid Shampoos Are Presumably Used by the Vast Majority of Consumers and Hairdressing Salons. Comparing these with solid shampoos, two highly significant differences are evident with regard to preservation: (i) the share of solid shampoos not preserved was significantly higher, (ii) the number of preservatives used was significantly lower in solid shampoos, notwithstanding occasional similarity as with regard to benzyl alcohol, piroctone olamine or behentrimonium chloride. Thus, on a general level, the initial hypothesis that solid shampoo should offer a lower exposure to preservatives of hairdressers has been confirmed. This picture is largely unchanged if use of “antimicrobials” is also considered, with about 60% of solid shampoos containing neither class of ingredients.

Looking at the specific preservatives used, those included in solid shampoo are relatively weak allergens. This includes the top‐ranking benzyl alcohol, which is undoubtedly an established contact allergen, but a rare and weak one [10]. By contrast, in liquid shampoo, several preservatives known to be potent contact allergens in humans have been found to be present [11]. This includes MCI/MI as well as MI. Their current restriction by the Cosmetics Regulation (EC 1223/2009) with a maximum permitted in‐product concentration of 15 ppm is intended to be safe as regards induction of sensitization by cosmetics. This has, however, not yet been demonstrated. Elicitation in the significant number of those sensitised to MI, especially by often‐repeated exposure of unprotected skin, is not unlikely, as illustrated by a repeated open application test (ROAT) study using 50 ppm MI five times daily in MI‐sensitised patients [12]. Hence, for MI sensitised individuals, avoidance of exposure by use of e.g., solid shampoo is an important option.

4.2. Limitations

Some general limitations of this type of “population exposure analysis” relying on the INCI labelling of cosmetic products should be mentioned. First, the “convenience” sample, while broad, may not be entirely representative for actual exposure, as often‐used products cannot be weighed for their contribution to overall exposure, relative to less often used products. Second, INCI labelling may not always be correct, compared to chemical analysis, which may explain that some, but presumably not all, solid shampoos containing water seemingly do not contain preservatives. This concerns also products even with a relatively high share of water, as evidenced by a high rank in the INCI declaration. The INCI declaration presents ingredients sorted by decreasing concentration, down to a level of 1% below which there is no strict convention regarding the order. However, we would not expect that such incorrectness should severely bias our observations. Thirdly, we do not have actual quantitative information on the share of various ingredients in solid shampoos (as for cosmetic products in general), including preservatives, which could be used for risk assessment. In this context, the concentration limits mentioned in Tables 1 and 4 are maximum values.

Specific to the product group in focus, in a few solid shampoos the use of decoctions or fresh juices may have effectively replaced the use of water, thus leading to some underestimation of the occurrence of water and its relation with preservation.

Finally, in the present study, we focused solely on preservatives (and “antimicrobials”) and not other allergens such as fragrances, for which no unequivocal evidence concerning an increased risk in hairdressers compared to consumers has been found [13, 14]. Independently, fragrances and other ingredients may generally differ between liquid and solid shampoos, which is difficult to take into account in the assessment of overall contact sensitization risk related to the use of shampoo.

4.3. Conclusion

Based on findings from this study, use of solid shampoos can result in reduced exposure to preservatives, particularly to those associated with a higher sensitization hazard and subsequent risk. It would, in our opinion, be desirable to further explore the applicability of solid shampoo in salons. This would concern domains such as ease of use, acceptability to clients, and adequate hygiene standards.

With regard to risk assessment, an exposure study on the amount of product typically applied by one wash with a solid shampoo should be performed to supplement information used for risk assessment such as by the Scientific Committee on Consumer Safety (SCCS); see “Notes of Guidance” [15], Table 3. Independent from the intended health effect for hairdressers, the use of solid instead of liquid shampoo could also generate less waste and lower ecological impact.

Author Contributions

Wolfgang Uter, Anders B. Funch, Mikkel Bak Jensen, Christoffer Kursawe Larsen, and Ying X. Teo: conceptualization, data curation, investigation, methodology, project administration, resources, validation, writing – original draft, writing – review and editing. Jeanne D. Johansen: conceptualization, methodology, resources, supervision, writing – original draft, writing – review and editing. Carola Lidén and Ian R. White: conceptualization, methodology, resources, supervision, writing – review and editing.

Conflicts of Interest

W.U. receives research funding directed to the department from IFRA (https://ifrafragrance.org/). The other authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.