Allergic Contact Dermatitis After Use of Headphones: A New Case of Contact Allergy to Dimethylfumarate (DMF)

Abstract

Dimethylfumurate (DMF; CAS: 624-49-7) is an antifungal agent [1]. DMF used in Chinese-manufactured recliners and sofas led to a significant outbreak of allergic contact dermatitis across Europe, commonly referred to as “toxic sofa dermatitis”. The first reported cases emerged in Finland in 2006 [1, 2]. Subsequently, a DMF content above 0.1 mg/kg was banned in consumer products in the European Union (EU) in 2009 [3]. Fifteen years after this ban, we report a Danish case of allergic contact dermatitis probably due to DMF exposure from headphones.

A 22-year-old man with keratosis pilaris was referred for patch testing due to bilateral periauricular red pruritic eczema (Figure 1). The eczema developed after the patient wore headphones during a two-hour flight (Sony WH-1000XM5–Black). Prior to this, the patient had only used the over-ear headphones a couple of times. A few months before the flight, the patient had experienced the same symptoms 1–2 days after exercising with the headphones on.

In July 2024, the patient was patch tested with the European baseline series, the supplementary baseline series at Gentofte Hospital, a fragrance series and the ear pads of his own headphones (Sony WH-1000XM5–Black; occluded with 12-mm Finn chamber and tested “as is”). Allergens were supplied by Allergeaze allergens (Smart Practice, Phoenix, Arizona, USA) or Chemotechnique Diagnostics (Vellinge, Sweden). Allergens were occluded with 8-mm Finn chambers (Smart Practice) to the upper back using Scanpor tape (Norgesplaster, Alpharma, Vennesla, Norway) for 48 h. Reactions were read day(D)2, D5 and D7. Patch testing was conducted according to the ESCD-guidelines [4]. The patient had a doubtful reaction to textile dye mix (6.6% pet.) (D2) and a positive reaction (++) to the headphones (D5). In January 2025, a second patch test was performed with a supplementary rubber series and a textile series. The patient had a positive reaction (++) to DMF (0.01% pet.) (D2), a doubtful reaction to disperse yellow 3 (D2) and disperse red 3 (both 1.0% pet.) (D5) and an irritant reaction to 4-Aminophenol (1.0% pet.) (D5).

The eczema resolved after treatment with hydrocortisone-17-butyrate. The patient changed his headphones to a different brand and has never had any problems since.

In the present case, the patient had positive reactions (++) to both his own headphones and DMF. We asked the manufacturer for the ingredient list of the headphones without success. However, according to the Danish retailer, the patient's headphones were made in Malaysia. DMF may be added to the product as an antifungal agent for shipping to Europe: For example, DMF has been found in “MouldProof” sachets that are often attached to products and which caused the toxic sofa dermatitis crisis [2, 5]. The patient also found sachets together with his earphones. Therefore, we concluded that the headphones were the probable relevant source of DMF exposure. However, since we do not have an ingredient list from the supplier and without a chemical analysis, we cannot conclude with certainty that the headphones were the exposure that caused the patient's contact allergy to DMF based on relevance. Hypothetically, the headphones could have contained dibutyl fumurate, which cross-reacts with DMF. This cannot be ruled out without a chemical analysis.

Local lymph node assays have shown that DMF is a strong sensitizer [1]. Combined with extensive exposure and the occlusive effect of furniture, this may help explain the severity of the toxic sofa dermatitis crisis [1]. Similarly, headphones are occlusive and worn for long periods of time, which may increase the risk of contact allergy to DMF.

A recent study identified octylisothiazolinone (OIT) in earphones as the cause of allergic contact dermatitis [6]. However, in the present case, the patient had a negative patch test reaction to OIT.

Following the EU ban, contact allergy to DMF has also been observed in other EU countries [7] and has been found in shoes in non-EU countries such as Argentina [8]. As stated by Basketter et al., “legislation is pointless unless it is properly applied and policed” [1]. The present case of contact allergy to DMF highlights this concern, regardless of whether the patient was sensitised through the headphones or another source. Manufacturers must ensure that their products comply with EU regulations, and governmental institutions should enforce these regulations to protect public health and maintain trust in authorities. Consumers have the right to know what they are exposed to through everyday products. Additionally, clinicians should remain aware of DMF as a potential cause of allergic contact dermatitis.

Christoffer Kursawe Larsen: writing – original draft, project administration, conceptualization, investigation, methodology, validation. Malin Glindvad Ahlström: writing – review and editing, investigation, supervision, conceptualization, methodology, validation. Jakob F. B. Schwensen: writing – review and editing, supervision, conceptualization, investigation, methodology, validation.

The patient has given written informed consent to publication of the case details and photographs.

The authors declare no conflicts of interest.