7 Materials in headphones and hearing protection aids

Survey of chemical substances in headphones and hearing protection aids

7.1 Materials in headphones
- 7.1.1 Materials, found via Internet searches
- 7.1.2 Information from producers
7.2 Materials in hearing protection aids
- 7.2.1 Materials, found through Internet searches
- 7.2.2 Producer information
7.3 Summary – survey of materials

7.1 Materials in headphones

The materials being used in headphones are mapped through Internet searches and via information from the producers.

7.1.1 Materials, found via Internet searches

Most of the information about materials is found via searches on the producers’ homepages on the Internet. Only a very limited part of the available products is described with regard to the materials coming into contact with the ear and in nearly all cases the information concerns the material being used for ear pads, or in which the loudspeakers in In-ear headphones are moulded.

Below the material being identified via the searches are stated, no matter if they appear often or are only found in a single case.

Table 3. Material identification in relation to headphones.

Material identification	Type of headphone
Silicone (incl. “medical grade” and “Hypoallergent”)	In-ear
Foam	On-ear
Leather-effect	On-ear
PVC (also “vinyl”)	On-ear
Thermoplastic polyurethane	Around-ear
PC (polycarbonate)	On-ear
Artificial leather (also “foam-filled artificial leather”	Around-ear
Rubber	In-ear
Textile (velour, velvet, plush)	Around-ear
”Slow-memory material”	Around-ear
Polyester (PET)	Ear cup for On-ear
Rubber	Ear hook for On-ear
Leather	Around-ear

Based on the searches it has not been possible to establish an unambiguous connection between the choice of material and the quality/price. Artificial leather (which is not a well-defined material) is together with PVC the material being mentioned most often for Around-ear headphones while textile and leather are only mentioned a few times, all in connection with products which are judged to be “high-end” products of a high quality. “Foam” is assumed to cover foam rubber which is primarily manufactured from polyurethane but it can also be manufactured from PVC.

For In-ear headphones silicone seems to be the dominant choice of material but also “rubber” is mentioned in this connection, however, without a more precise description. However, it must be assumed that In-ear headphones with “hard” ear buds are not manufactured from silicone but from plastic types like (hard) PVC (polyvinylchloride), PC (polycarbonate) or ABS (acrylic onitrile-butadiene-styrene). For many of these, a cover of foamed plastic is included which in the same way as for Around-ear and On-ear headphones will primarily be manufactured from PUR (polyurethane) or PVC.

7.1.2 Information from producers

The inquiries to producers of headphones confirmed that the preferred materials for use in headphones are PVC, polyurethane and silicone for Around-ear and on-ear headphones while the soft material being applied in in-ear headphones is silicone. Which materials are being applied for in-ear with hard ear-buds did not appear from the received answers.

The answers from producers/suppliers only contributed with few details besides information about the primary materials being applied. However, one single producer of head sets and other electronic equipment placed an analysis report of a product at our disposal. From the report and the following communication it appears that the following materials can get into contact with the skin for the product in question:

EVA (ethylene vinyl acetate) in ear pads
A mixture of PP (polypropylene) and EPDM (ethylene/propylene dimer), a so-called thermoplastic elastomer, in an ear hook
ABS (acrylic onitrile-butadiene-styrene), in a fastening spring band
Soft foam, manufactured from chloroprene rubber for a stabilization spring band
PC/PBT (polycarbonate/polybutylene terephthalate), for loudspeaker house

A detailed examination of the analysis results is found in section 8.3.

7.1.2.1 Information about toxicology and user-experience

None of the producers of headphones answering to the inquiry had the knowledge that their products had caused allergy or other inconveniences and diseases for the users. It shall be noted that information about this from individual persons cannot be regarded as fully adequate but it has been outside the frames of the project to follow up at producers and suppliers.

An acknowledged institute has assessed the analysis results of the headset being described in general in 7.1.2 and more detailed in section 8.3 with regard to the risk that the materials cause allergy in the users of the headset. In their assessment, the institute points at the following substances as potentially problematic in relation to allergy: cobalt, nickel, chromium and 4-tert-butyl phenol. Based on the low concentrations of the four substances in the materials, the institute concludes that allergic reactions cannot be expected. None of the substances identified in the analysis were regarded as irritants. However, the institute remarks that there might have been other substances in the materials than those being selected for the analyses.

7.2 Materials in hearing protection aids

Mapping of the materials being used in hearing protection aids was attempted through Internet searches and direct inquiries to producers.

7.2.1 Materials, found through Internet searches

Through the Internet searches the following materials were identified for the two types of hearing protection aids.

Table 4. Materials in hearing protection aids

Around-ear	In-ear
PVC (in a bag, not detailed described)	Silicone Medical silicone (approved for implants) Acrylic Acrylic/silicone-mixture

7.2.2 Producer information

The occurrence of PVC in Around-ear hearing protection aids was confirmed by three of the large producers. One of them also told that two tests for allergenic properties had been made – both were negative. Unfortunately, it has not been possible to get further details about the content of additives in the products in question or about the allergy tests in question. However, another producer provided a list of the requirements to concentrations of substances which their products must comply with in order to be approved by BG Prüfzert which is a German association of test and certifying bodies which primarily deals with working environment. For hearing protection aids the following requirements to constituents are laid down:

Table 5. Requirements in BG Prüfzert for maximum concentrations of certain substances and groups of substances

Substance/group of substance	Concentration
Metals
Arsenic	≤ 25 mg/kg
Lead	≤ 25 mg/kg
Antimony	≤ 25 mg/kg
Tin	≤ 50 mg/kg
Phthalates
Di-(2-ethylhexyl)-phthalate	≤500 mg/kg
Total phthalate Dimethylphthalate, diethylphthalate, di-n-butylphthalate, di-isobutylphthalate, benzylbutylphthalate, di-(2-ethylhexyl) phthalate, di-n-octyl-phthalate, di-iso-nonylphthalate, other phthalates being quantifiable with reference to the applied method of analysis.	≤ 5000 mg/kg (until 30/6-2008 ≤ 7500 mg/kg)
Organisc tin compounds (Total of monobutyl, dibutyl, tributyl, tetrabutyl and dioctyl tin. Shall only be determined if the concentration of tin is larger than 50 mg/kg).	= 50 mg/kg

It is outside the frames of the project to complete an assessment of the importance of fulfilling these requirements. However, it is clear that for phthalates it is rather low concentrations that are permitted (0.05% di-(2-ethylhexyle)phthalate and 0.5% total phthalate) as soft PVC often contains more than 30% phthalate.

Likewise the use of silicone and acrylic in in-ear hearing protection aids was confirmed by suppliers. For in-ear hearing protection aids of silicone both raw materials and a “finished” set of hearing protection aids were made available for analyses. A producer of in-ear hearing protection aids of both silicone and acrylic made a set of safety data sheets available for the project. It should be noted that in-ear hearing protection aids of both acrylic and silicone are individually fitted as they are either manufactured through embedment directly in the ear (possible with silicone) or embedment in a mould giving an imprint of the shape of the ear. The safety data sheets describe the following substances which are the basis of classification of the raw materials:

Table 6. Constituents in raw materials for production of hearing protection aids in silicone and acrylic. From the producers’ safety data sheets.

Hearing protection aids of silicone	Constituent(s)	CAS No.	Concentration
	Cristobalite	14464-46-1	10-25%
	Paraffin waxes and carbon hydride waxes	8002-74-2	Working place exposure possible – concentration not stated
	Amorphous silica	07631-86-9	25%
	Dimethyl, methylhydrogene siloxane copolymer	68037-59-2	10%
Hearing protection aids of acryl
	Methylmethacrylate	80-62-6	50-100%
	Ethandiol-1,2,-dimethacrylate	97-90-5	2.5-10%
	2-Benzotriazolyl-4-methylphenol (Tinuvin P)	2440-22-4	≤ 2.5%
	N,N-dimetyl-p-toluidine	99-97-8	≤ 2.5%
	1,4-dihydroxybenzene	123-31-9	≤ 0.1 %
	Dietylphthalate	84-66-2	Working place exposure possible – concentration not stated

At least one producer states to have a possibility of surface treatment of hearing protection aids of silicone with a lacquer based on toluene (20-60%) and xylene (20-60%).

From another producer of hearing protection aids of silicone a bottle with lubricant is included which can ease the insertion of the hearing protection aids. Among the constituents in this lubricant is phenyl mercury nitrate in concentrations of less than 0.01%. The substance is a preservative.

7.2.2.1 Information about toxicology and user-experience

None of the producers of Around-ear hearing protection aids answering to the inquiry had the knowledge that their products had caused allergy or other inconveniences and diseases for the users. It shall be noted that information about this from individual persons cannot be regarded as fully adequate but it has been outside the frames of the project to follow up at producers and suppliers.

However, a producer of both hearing aids and hearing protection aids informed that their experience with hearing aids in hard acrylic was that about 2% of the users experienced allergenic skin rwactions without stating information about the specific reason for the allergy. To these patients the hearing aids were therefore produced in titanium or another hypoallergenic material. Apparently, the producer in question does not manufacture hearing protection aids in hard acrylic.

As mentioned in 7.2.2 a producer of Around-ear hearing protection aids with a bead (bulge/thickening) of PVC has carried out allergenic tests of their products with a negative result. It has not been possible to get further details.

A producer of In-ear hearing protection aids of silicone has carried through tests of biological reactivity of typical raw materials. The following results are described:

Systemic toxicity in mice. Intraperitonal og intracutaneous administration in mice caused no larger systemic reaction from the test preparation than from salt water, polyethylene glycol 400 or sesame oil.
The macroscopic reaction from implantation in the muscles of a rabbit was not significant, compared with an USP negative control plastic.

The tests thus show primarily that there is no biological reaction from contact with silicone.

7.3 Summary – survey of materials

First of all, the completed survey shows that a wide range of materials is applied in headphones. The main part of the materials is synthetic polymers (plastic and rubber) but also a natural material like leather is used. It shall be noted that “textile” in this connection is assumed to be polymer-based, e.g. in the form of nylon or polyester.

For hearing protection aids the survey shows that PVC apparently is the preferred material for Around-ear hearing protection aids while silicone and acrylic are the preferred materials for In-ear hearing protection aids.