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Survey of chemical substances in headphones and hearing protection aids

8 Analysis of selected products

8.1 Purchase strategy

The preliminary survey of the materials and possible additives which can come into contact with the skin has been supplemented with an analysis of a number of headphones and hearing protection aids, chosen randomly without possibility of determining whether the selection is representative. Within the product group headphones alone, more than 1,000 products are available on the Danish market and the selection of products for analysis took therefore preliminary place with basis in criteria such as the largest suppliers were to be represented, that the three types of headphones (Around-ear, On-ear and In-ear) and two main types of hearing protection aids (Around-ear and In-ear) were to be included, that the surfaces as far as possible were different (e.g. foamed/smooth surface and hard/soft in-ear headphones), and at the same time that the selected products were to be in different price levels.

It shall be noted that the differences in the price do not necessarily reflect differences in the choice of material but rather that there is a better sound quality in the more expensive products. It shall also be noted that no products with a price above approx. 400 DKK were purchased. Thus, some products on the market cost above 2,000 DKK, a price which only to a very limited degree can be assumed to be related to the choice of exclusive materials for contact with the skin but rather to the use of exclusive materials (e.g. gold) for cord and generation of the sound signals.

The relevant characteristics for the purchased products are:

Table 7. General characteristics of the purchased products.

Product group Number of products Number of producers Prices
Around-ear headphones 6 6 118, 134, 230, 273, 384, 771 DKK
On-ear headphones 4 4 20, 189, 384 DKK
In-ear headphones 4 4 74, 119, 175 DKK
Around-ear hearing protection aids 4 4 58, 130, 159, 169 DKK
In-ear hearing protection aids 2 2 Free

The majority of the purchased products (11/14 headphones, 2/6 hearing protection aids) was purchased via the Internet. The primary reason for this was that in this way it was relatively easy to secure that the purchased products were distributed as wanted. For the hearing protection aids, the purchases were spread on two products from do-it-yourself markets, two from the Internet and two products which the producers delivered free of charge to the project, one as a sample, the other being produced on site.

8.2 Analysis results

There were two main targets of the analyses, i.e. to verify the producer information and to complement these. However, in practice it was not possible to verify the producer information as the purchased products were not provided with information about which materials being applied. Exceptions from this were the two In-ear hearing protection aids where both producers clearly tell which materials form the basis of the products.

8.2.1 Analysis method

The IR analyses were completed on a Nicolet Impact 400 FT-IR spectrometer.

Different techniques were applied, dependent on the product. Flat, smooth materials were examined by use of ATR technique. Materials which were not smooth and flat were examined by rubbing a silicon carbide sandpaper against the sample and record the spectrum by DRIFT (diffuse reflectance) with the clean sandpaper as reference.

ATR as  well as DRIFT are reflection techniques and the spectra become a little distorted compared to normal transmission spectra.

For identification of plastic types electronic reference libraries (Hummel-Scholl eller Sadtler Know-it-all) were mainly used combined with FORCE Technology’s general experience.

Plasticizers like phthalates are normally applied in large amounts (30%) and will immediately be seen in the spectra. Often these substances will camouflage the spectrum of the basic polymer. Phthalate being present in a few percentages of another ester will normally not be observed. Other additives being applied in 0.1 to a few percentages will normally not be observed at the screening analysis unless they have absorptions in areas where a basic polymer and a possible softener for certain do not absorb.

Fillers with characteristic spectra, e.g. chalk, will be identified at levels of 10-30% while other fillers often cannot be identified for certain.

8.2.2 Analysis results

The results of the analysis of the selected products are shown in Table 8.

Table 8. Identified materials at FTIR analysis.

Product description IR result + remarks IR technique Product no.
Headphones      
Headphone Around ear Polyurethane (soft) ATR 5
Headphone Around-ear Polyurethane (soft) ATR 7
Headphone Around-ear Polyurethane (soft) ATR 14
Headphone/headset Around-ear Polyurethane (soft) ATR 16
Headphone In-ear Silicone (polydimethyl siloxane) ATR 6
Headphone In-ear Foam part: Polyurethane
hard plastic: ABS 		ATR- foam + DRIFT hard part 11
Headphone In-ear Polybutadiene-styrene ATR 12
Headphone In-ear ABS DRIFT 18
Headphone On-ear Polyurethane (soft) ATR 4
Headphone On-ear Polyurethane (foam) ATR 8
Headphone On-ear Polyurethane (soft) ATR 9
Headphone On-ear Very black textile material, probably a typical polyester (PET) ATR 10
Headphone On-ear Polyurethane (soft) ATR 13
Headphone/headset On-ear Polyurethane (foam) ATR 17
Hearing protection aids      
Hearing protection aids Around ear Polyurethane (soft) ATR 1
Hearing protection aids Around ear PVC/Nitrile with unidentified plasticizer and/or flame retardant (probably containing phosphorus, similar to product no. 19) ATR 2
Hearing protection aids Around-ear PVC with phthalate plasticizer ATR 15
Hearing protection aids Around-ear PVC/Nitrile with unidentified plasticizer and/or flame retardant (probably containing phosphorus, similar to product no. 2) ATR 19
Hearing protection aids In-ear Silicone (polydimethyl siloxane) ATR 3
Hearing protection aids In-ear Polyurethane, hard (surface/lacquer) DRIFT 20

In general, the results confirm the information being found in the mapping phase of the project, namely that PUR (polyurethane) in both foamed and soft (artificial leather) quality is often used in parts getting into contact with the ear (On-ear and Around-ear products), that silicone is applied in In-ear products (both headphones and hearing protection aids), and that PVC is a frequently used material in Around-ear hearing protection aids.

Furthermore, the analysis showed some elements which did not appear from the preliminary survey:

  • Polyurethane based lacquer is applied for surface treatment of In-ear hearing protection aids in a hard quality, probably acrylic. According to the producer, the lacquer reduces the risk of allergy.
  • PVC applied in Around-ear hearing protection aids can be modified with nitrile (acrylonitrile-butadiene rubber) which increases the resistance towards oil and different chemical substances and it also gives an increased flexibility in the material. Thus, nitrile rubber can possibly replace traditional softeners totally or partly.
  • For PVC applied in Around-ear hearing protection aids, also traditional plasticizers are used. Unfortunately, the analysis does not give a possibility of a more precise identification of which plasticizer is used.
  • In the two In-ear headphones where the hard material was analysed, this was ABS. In the one of the cases the user could choose to put on pads of polyurethane, while the other product did not give this possibility.
  • Polybutadiene styrene (a type of rubber) is used as (soft) material in In-ear headphones.

It is emphasized again that the examined products do not necessarily constitute a representative sample of the two product groups. It shall also be emphasized that it was not within the frames of the project to quantify the content of additives in the products; in some cases it was even impossible to determine the substance(s) in question due to interference.

The existing knowledge can be summarized in a few points:

  • Which hard materials get into contact with the ear?
    • It is known for certain that ABS is used but there are no technological barriers for using materials like polycarbonate (PC), possibly as a copolymer with ABS or PBT. Also, there are no technological barriers for using hard PVC or PP for the purpose.
  • Which soft materials get into contact with the ear?
    • There is no doubt that PUR is the preferred choice for ear pads for all kinds of headphones, both in the form of foam pads and smooth pads. The analyses do not show whether it is a “pure” PUR material or it is an artificial leather quality where the contact layer is PUR while the substrate layer might be a different plastic or textile (se section 9.3).
    • From the detailed analysis of a headset it is known that also foam, based on neoprene rubber, can be used. PVC foam is also a possibility but no information is found showing that this material is used in practice.
    • Only a single of the analyzed products had a textile surface in contact with the skin. This material was most probably polyester (PET) which is a well-known and often applied material in all kinds of textiles. As described in section 9, it other synthetic textiles can most probably also be used.
  • Which plasticizers are used in PVC – and in which amounts?
    • Most probably soft PVC being used in headphones and hearing protection aids is modified in the same way as other soft PVC products. In other words, a wide range of plasticizers (including phthalates) can be used in amounts up to 40%.
  • Are flame retardants applied?
    • In two Around-ear hearing protection aids, flame retardants of the phosphate type might be applied. It is unknown whether this is the case for other hearing protection aids.
  • For none of the identified materials is it possible on basis of the analysis to give a qualified suggestion about type and amount of the additives used. Therefore, typical compositions of frequently used materials given in section 9.

8.3 Analysis of materials in a headset

A producer of headsets and other electronic equipment made an analysis of the materials being a part of one of there headsets available to the project. The headset contains number of components which in one way or another can get into contact with the skin.

  • Ear pads in two sizes, both produced from EVA (ethylene vinyl acetate), with contact surface at the external ear.
  • Ear hook, produced from a mixture of PP (polypropylene) and EPDM (ethylene/propylene dimer), a so-called thermo plastic elastomer, with a contact surface on the back of the ear.
  • Fastening spring band/ear plate, produced from ABS (acrylonitrile-butadiene-styrene), with a contact surface on the external ear.
  • Stabilization spring band with soft foam, produced from neoprene rubber, with contact on the scalp.
  • Loudspeaker house, produced from PC/PBT (polycarbonate/polybutylene terephthalate), with a (small) contact surface on the external ear.

The headset being analyzed can be varied in different ways. For instance, it is possible to choose between a large and a small ear pad and it can be fastened in three different ways by means of spring bands which either is placed above the scalp or around the ear. The basic model is shown in Figure 6.

Figure 6. The basic design of the analyzed headset.

Figure 6. The basic design of the analyzed headset.

The analysis results for the materials getting into contact with the skin are shown in Table 9 and Table 10. The analyses was made by an accredited laboratory but the information about analysis methods and detection limits has not been available.


Table 9. Concentration of metals and chemical compounds getting into contact with the skin in the analyzed headset.

Component   Reciver house w. loudspeaker Ear pad, small Earpiece Ear pad, large
Material   PC/PBT EVA ABS EVA
Chemical compound Concentration        
As mg/kg <0.1 <0.6 <0.1 <0.2
Cd mg/kg 0.17 <0.04 4.14 <0.01
Co mg/kg 0.0173 0.64 0.298 0.33
Cr mg/kg 0.12 991 0.458 810
Hg mg/kg <0.02 <0.08 <0.02 <0.02
Mn mg/kg 0.14 0.399 0.255 0.4
Ni mg/kg 0.167 0.429 3.49 0.309
Pb mg/kg 0.0875 <0.3 0.35 0.101
Zn mg/kg 22 2.22 39.2 1.5
Dimethyl phthalate mg/kg <1.0 <1.0 <1.0 <1.0
Diethyl phthalate mg/kg <1.0 <1.0 <1.0 <1.0
di-n-propyl phthalate mg/kg <1.0 <1.0 <1.0 <1.0
di-isobutylephthalate mg/kg <1.0 18 <1.0 8.5
di-n-butyl phthalate mg/kg <1.0 20 <1.0 6
di-pentyl phthalate mg/kg <1.0 <1.0 <1.0 <1.0
di-n-octylphthalate mg/kg <1.0 <1.0 <1.0 <1.0
di-(2-ethylhexyl)phthalate mg/kg <1.0 <1.0 <1.0 4.3
Butylbenzyl phthalate mg/kg <1.0 <1.0 <1.0 <1.0
di-cyclohexyl phthalate mg/kg <1.0 7 <1.0 <1.0
di-isononyl phthalate mg/kg <10 12 <10 <10
di-isodecyl phthalate mg/kg <10 <10 <10 <10
4-tert-butylphenol mg/kg 0.29 0.66 0.26 0.79
4-tert-pentylphenol mg/kg <0.0050 0.11 <0.0050 0.013
4-tert-octylphenol mg/kg <0.0050 0.16 <0.0050 0.062
4-n-nonylphenol mg/kg <0.0050 <0.0050 <0.0050 <0.0050
iso-nonylphenol mg/kg <0.050 53 <0.050 7.5
bisphenol A mg/kg 3.2 0.9 1.9 0.13
octylphenol-monoethoxylate mg/kg <0.010 <0.010 <0.010 <0.010
octylphenol-diethoxylate mg/kg <0.010 <0.010 <0.010 <0.010
octylphenol-triethoxylate mg/kg <0.010 <0.010 <0.010 <0.010
octylphenol-tetraethoxylate mg/kg <0.010 <0.010 <0.010 <0.010
octylphenol-pentaethoxylate mg/kg <0.010 <0.010 <0.010 <0.010
octylphenol-hexaethoxylate mg/kg <0.010 <0.010 <0.010 <0.010
nonylphenol-monoethoxylate mg/kg <0.10 <0.10 <0.10 0.85
nonylphenol-diethoxylate mg/kg <0.10 <0.10 <0.10 1.4
nonylphenol-triethoxylate mg/kg <0.10 <0.10 <0.10 5.6
nonylphenol-tetraethoxylate mg/kg <0.10 <0.10 <0.10 11
nonylphenol-pentaethoxylate mg/kg <0.10 <0.10 <0.10 17
nonylphenol-hexaethoxylate mg/kg <0.10 <0.10 <0.10 23


Table 10. Concentration of metals and chemical compounds getting into contact with the skin in the analyzed headset (continued).

  Component T-piece, selv-adhesive foam Ear spring band, soft Ear plate with adaptor
Material   Chloroprene PP/EPDM ABS
Chemical compound Concentration      
As mg/kg <3 <0.3 <0.1
Cd mg/kg <0.1 0.102 <0.008
Co mg/kg <0.1 0.0648 <0.008
Cr mg/kg 1.72 0.897 <0.05
Hg mg/kg <0.2 <0.04 <0.02
Mn mg/kg 1.67 9.9 <0.07
Ni mg/kg <0.9 8.08 <0.07
Pb mg/kg 2.22 0.242 <0.07
Zn mg/kg 13200 522 18.2
Dimethyl phthalate mg/kg <1.0 <1.0  
Diethyl phthalate mg/kg <1.0 <1.0  
di-n-propyl phthalate mg/kg <1.0 <1.0  
di-isobutylephthalate mg/kg 4.1 2.4  
di-n-butyl phthalate mg/kg 1.3 <1.0  
di-pentyl phthalate mg/kg <1.0 <1.0  
di-n-octylphthalate mg/kg <1.0 <1.0  
di-(2-ethylhexyl)phthalate mg/kg 87 5.7  
Butylbenzyl phthalate mg/kg <1.0 <1.0  
di-cyclohexyl phthalate mg/kg <1.0 <1.0  
di-isononyl phthalate mg/kg <10 <10  
di-isodecyl phthalate mg/kg <10 <10  
4-tert-butylphenol mg/kg 0.15 <0.0050  
4-tert-pentylphenol mg/kg <0.0050 <0.0050  
4-tert-octylphenol mg/kg <0.0050 <0.0050  
4-n-nonylphenol mg/kg <0.0050 <0.0050  
iso-nonylphenol mg/kg 0.46 1.3  
bisphenol A mg/kg <0.050 6.4  
octylphenol-monoethoxylate mg/kg <0.010 <0.010  
octylphenol-diethoxylate mg/kg <0.010 <0.010  
octylphenol-triethoxylate mg/kg <0.010 <0.010  
octylphenol-tetraethoxylate mg/kg <0.010 <0.010  
octylphenol-pentaethoxylate mg/kg <0.010 <0.010  
octylphenol-hexaethoxylate mg/kg <0.010 <0.010  
nonylphenol-monoethoxylate mg/kg <0.10 <0.10  
nonylphenol-diethoxylate mg/kg <0.10 <0.10  
nonylphenol-triethoxylate mg/kg <0.10 <0.10  
nonylphenol-tetraethoxylate mg/kg <0.10 <0.10  
nonylphenol-pentaethoxylate mg/kg <0.10 <0.10  
nonylphenol-hexaethoxylate mg/kg <0.10 <0.10  

Besides the above-mentioned substances, analyses for the following chemical compounds were carried out:

  • anisidine
  • 4-aminobiphenyl
  • benzidine
  • 4-chlor-o-toluidine
  • 2-naphtylamine
  • p-chloraniline
  • 4-methoxy-m-phenylendiamine
  • 4,4-diaminodiphenylmethane
  • 3,3-dichlorobenzidine
  • 3,3-dimethoxybenzidine
  • 3,3-dimethylbenzidine
  • 3,3-dimeth.-4,4-diaminophen.m.
  • 4,4-methylene-bis-(2-chl.an.)
  • 2-amino-4-nitrotoluen
  • 4,4-oxydianiline
  • 4,4-thiodianiline
  • 4-methyl-1,3phenylendiamine
  • 2,4,5-trimethylaniline
  • 2-methoxy-5-methylaniline
  • toluidine

None of these substances was found in concentration above the detection limit.

8.3.1 Assessment of the analysis results

An acknowledged institute has evaluated the analysis results with regard to the risk of the materials causing allergy in the users of the headset. In their evaluation, the institute points at the following substances as potentially problematic in relation to allergy: cobalt, nickel, chromium and 4-tert-butylphenol. Based on the low concentrations of the four substances in the materials, the institute concludes that no allergenic reactions can be expected. None of the identified substances is considered to be irritants. However, the institute notes that there may have been other substances in the materials than those that have been selected for the analyses.

8.3.2 Comment to analysis results and assessment

Besides an assessment of the risk of allergy, the analysis results give a possibility of pointing out problematic substances in the materials being applied. However, the analyses give only information about the content of substances, not whether they can migrate migration to (artificial) sweat and thus can cause exposure of the users.

The substances in Table 11 are well-known problematic substances. In the table their concentration in the analyzed materials is stated as well as a rough estimate of the total weight of the substance in the components.

Table 11. Known problematic substances and their concentrations in the analyzed headset.

Component Material Substance Concentration (mg/kg) Weight of component (g) rough estimate Total weight of substance (microgram)
Earpiece ABS Cadmium 4.14 2 8
Earpiece ABS Lead 0.35 2 1
Loudspeaker house PC/PBT Bisphenol A 3.2 4 13
Ear hook PP/EPDM Bisphenol A 6.4 2 13
T-piece Neoprene-rubber DEHP (di-(2-ethylhexyl)phthalate 87 3 260

It is emphasized that the above calculation of the total weight of the given substances in the single components is a rough estimate, indicating the order of magnitude.

The total weight of the substances expresses how large the total exposure theoretically can be in the lifetime of the product, probably one year or more. In practise, the exposure is presumably much lesser as in order to be able to affect humans the substances must first be released from the plastic matrix in which they are fixed and then migrate through the skin. Even if the full weight of the substances in question migrates it is assessed that this will not give a risk of effects on the health and therefore it is judged that the concentration of the substances in question is not problematic.

Finally, it shall be mentioned that the analysis is judged to be representative of the materials being applied. However, it cannot in any way be excluded that in similar materials from other raw material suppliers other substances and/or other concentrations of substances can be found. Finally, it is emphasized that the choice of materials for the analyzed headset is not assessed to be particularly representative for the total market for headphones and hearing protection aids, but that the choice still reflects some of the possibilities which are available for a producer.

 

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Version 1.0 April 2008, © Danish Environmental Protection Agency