Endocrine Disruption

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Health and environmental effects

Endocrine disruptors (EDs) are implicated in declining sperm counts, genital malformations in newborns, and rising rates of certain cancers. There are also possible links with impairment in neural development and sexual behaviour.

In wildlife, EDs can cause hermaphroditism – a condition known as intersex [1].

EDs interact with the hormone system of both humans and wildlife to cause adverse health effects. They have been difficult to regulate, because of their variety of modes-of-action, and sometimes complex or delayed effects.

REACH [2] Article 57 mentions substances with ‘endocrine disrupting properties’ as being of equal concern as carcinogens, mutagens and reproductive toxicants (CMRs), and are therefore also subject to Authorisation and forced withdrawal from the market.

This article summarises recent regulatory work in defining what counts as an ED, and the programme of work to assess chemicals for this hazard.

Endocrine system

The endocrine (or hormone) system regulates many important aspects of human and animal life, such as sexual development, changes during pregnancy and puberty, libido, menstruation, sperm count, and some behaviours.

The endocrine system is sensitive to chemical exposure. If adverse effects occur, they are in many cases irreversible and stay with the affected person or organism for the rest of its life.

The most widely studied endocrine effects are related to only three hormone types: estrogens (in female sexual development and reproduction), androgens (particularly testosterone in male development), and thyroxine (in control of reproduction and metabolic rate). Dozens of other hormones have not been properly assessed as targets for EDs.

Endocrine Disrupters (EDs)

An internationally accepted definition of EDs is give by the World Health Organisation (WHO/IPCS): ‘an endocrine disruptor is an exogenous substance or mixture that alters functions(s) of the endocrine system and consequently causes adverse health effects in an intact organism, or its progeny, or (sub)populations.’

The first feature of the definition is that of ‘adverse effect’ in an intact organism, ie not merely a biochemical interaction such as receptor binding, or even a hormone-modulating effect. Many chemicals show weak binding with hormone receptors, and might be termed as having endocrine activity. But receptor binding may not lead to an adverse effect, either because the ED is not present at high enough concentrations in the ‘intact organism’ or because the endocrine system adapts to nullify the effect.

This raises a difficult regulatory question: when is the line of adversity crossed?

The definition also specifies a causal link between the chemical and the adverse effect. This can be difficult to prove because:

  • There might be long time between the exposure of the organism and the effect, so the chemical is not present when the effect is manifest.
  • EDs may manifest adverse effects only if the organism is exposed during certain sensitive stages of its lifecycle, particularly in the womb and during puberty.
  • The disruption of hormone signalling often produces a tell-tale collection of effects in organs (a fingerprint) rather than a single, measurable criterion
  • The endocrine system is complicated, and proof of causality requires specific understanding of exactly how the substance interacts with it, and the consequences.
  • Little is known about their endocrinology of most wildlife species, especially invertebrates, plants, bacteria.
  • Current REACH testing is not aimed at detecting EDs. Some higher level (Annexes IX and X) testing, particularly in the rat, may pick up some endocrine effects, but these tests are not particularly sensitive and are conducted only for a limited number of substances.

Note that REACH states that the ED must cause probable serious effects before it becomes a substance of very high concern (SVHC), which might be seen as requiring a lower burden of proof than the WHO definition above.

Evidence of harm from EDs

With the difficulty in proving a chemical substance to be an ED, there is growing acrimony between the chemical industry and some NGOs. Relatively few chemicals have sufficient data to determine whether they are EDs. How should the chemical industry respond without proper evidence for specific substances? How should the ‘precautionary principle’ be evoked for substances with less-than-conclusive proof?

While proving that a particular chemical should be regarded as an ED if difficult, there is mounting epidemiological evidence that current background levels of EDs in the environment are a serious concern:

  • There is a plausible role for exposure to EDs in breast, prostate, testicular and thyroid cancers.
  • There are reported declines in men’s reproductive health, particularly semen quality, and increased problems in boys of testicular formation and genital abnormalities.
  • Male fish exposed to sewage treatment works effluent can develop sex organ malformation and have impaired reproductive function.
  • Birds can suffer dramatic changes in reproductive behaviours, including increased incidences of homosexual pairings.
  • There are many examples of vulnerable life stages in wildlife species, including lobsters, amphibians and reptiles, that are extremely sensitive to the influences of EDs.

Common chemicals implicated as EDs

The last 10 years have seen increased concern regarding EDs, both in the number of chemicals implicated and the variety of interaction with the endocrine system. This concern is magnified by the identification of several ubiquitous chemicals as potential EDs.

Perfluorooctanoic acid (PFOA) is widespread at low levels in the environment and in the blood of the general population. It is used to make fluoropolymers, and it can be produced by the breakdown in the environment of some perfluorinated surface-treatment products. PFOA inhibits thyroid function and reduces thyroid hormone levels in both humans and experimental animals, and can cause irreversible neurological changes in the womb. In humans, researchers have linked polyfluoroalkyl substances (PFASs; see also Peristent Organic Pollutants) in blood levels and ADHD in children. Epidemiology shows an association between PFAS body burdens and increased cholesterol levels. The strength of the evidence is controversial.

Bisphenol A (BPA) is produced in large quantities for use in the production of polycarbonate plastics and epoxy resins, which have applications in food and drink packaging, baby bottles, medical devices, coatings for food cans, bottle tops, water supply pipes, and dental sealants. BPA can bind to hormone (estrogen and progesterone) receptors and can act as a thyroid hormone antagonist. Scientists have demonstrated that exposure to BPA during organ development may cause irreversible adverse effects on the prostate gland, and changes in mammary tissue. There are emerging potential risks for cancer and neurological development.

Phthalates are the most widely used plasticizers, and have been used for about 50 years to make polyvinyl chloride (PVC) soft, flexible, and durable. Other uses are for coatings, such as nail polish, adhesives, sealants, and paints. US research provides good evidence of irreversible genital traits in male babies (eg problems with testicular descent) associated with higher exposure to phthalates in the womb. Prenatal phthalate exposure is a possible risk factor in ADHD, lowered IQ, obesity and diabetes. In experimental animals, phthalates produce a series of irreversible effects in male offspring (phthalate syndrome), characterized by underdeveloped or malformed reproductive organs. These effects can be traced to interference with testosterone synthesis in fetal life.

Parabens are the most widely used preservatives in cosmetic products. They are emerging as compounds of concern. They are metabolised very quickly by the body, so their metabolites are likely to play a role in any endocrine disruption. In experimental animals, paraben administration has been shown to reduce testosterone levels in a dose-dependent manner and reduce sperm production. In vitro, parabens have been shown to have estrogenic and anti-androgenic properties in receptor-binding studies.

How much is too much?

The chemical industry, in response these claims and concerns, often counter that the effects are not relevant for realistic exposures. A basic toxicological principle is that the dose makes the poison, or in other words that all substances have detrimental effects if the organisms is given sufficiently high doses.

A key question for EDs is at what dose do the adverse effects occur in an intact animal?  For general health effects, and assessor can calculate a safe level of exposure for a substance – the Derived-No-Effect Level (DNEL), which is a crucial output of the REACH Chemical Safety Assessment, and is communicated down the supply chain in the safety data sheet.

However, some scientists and advocates of stricter control of EDs regard the traditional approach to chemical risk assessment as too simplistic, for the following reasons.

Cocktail or mixture effects

There is good evidence that several EDs can work together. Exposure to multiple chemicals with the same endocrine mechanism can produce adverse effects where the individual chemicals would show no effect. Some commentators have described the environment as a ‘sea of estrogens’.

Low-dose effects

Non-standard dose–response relationships have been observed with EDs, so that effects have occurred at low doses that would not have been predicted from high-dose data. Most standard testing (eg for REACH) is done using comparatively high doses, so some argue that important low-dose effects have been missed. Others hotly dispute the validity of this data.

Shortcomings of REACH Testing

REACH testing does not mandate tests that are specifically designed to assess endocrine disrupting ability. Long-term animal tests, such as those prescribed in REACH Annex IX and X, which can provide some information on EDs, are only required for substances supplied at high tonnages.

However, even these may not detect EDs for the following reasons:

  • Rats are poor animal models for hormonally mediated cancers in humans; there are no suitable animal models for prostate, testicular or thyroid cancers.
  • Mutagenicity tests are specifically designed for the detection of genotoxic substances, relevant for some carcinogens or reproductive toxicants, but not for detecting endocrine-mediated effects.
  • Developmental and reproductive testing has limitations for detecting EDs concerning the exposure period .
  • Relatively rare but significant ED effects may be missed because of the low number of animals used.
  • There is no requirement to assess endocrine activity in aquatic organisms in REACH testing.
  • Understanding of endocrinology is limited. For most wildlife species, validated test methods do not exist.

Specific tests for EDs

Standard chemical testing, for REACH registration are not sufficient to asses a chemical for endocrine disrupting properties.

Other evidence may come from structure–activity relationships, or in vitro tests relating to receptor binding.

If a chemical is suspected of being an ED, then there are an increasing number of specialised tests to investigate this possibility. Some of these tests are not internationally validated, so their use in regulatory decision making is limited.

The assessor needs to choose appropriate tests based on available information, in a weight-of-evidence approach.

The uterotrophic assay [3] is an in vivo assay for the estrogenic activity of a test chemical. The growth phase of the uterus is controlled by estrogens. The test often uses ovariectomized animals to remove endogenous estrogen, so that the growth of the uterus becomes sensitive to external sources of estrogenic substances.

The Hershberger assay [4] assesses the androgenic activity of a test chemical. The test substance is given to castrated male rats. These animals are sensitive to exogenous androgens. The assessor looks for changes in weight in androgen-dependent tissues, such as the prostate gland.

The amphibian metamorphosis assay [5] identifies substances that may interfere with the thyroid action. In the test, tadpoles are exposed to a test chemical in water for 21 days. The assessor looks for changes to the shape of the tadpole (eg length, limb length, weight, and developmental stage), and histology of the thyroid gland.

Regulatory control of EDs

The REACH Regulation aims to ensure a high level of protection of human health and the environment from hazardous chemical products. One way that REACH achieves this is to identify and restrict the use of substances of very high concern (SVHCs), which are defined in Article 57 of REACH, and include EDs.

SVHCs are subject to the Authorisation process. Substances on the list of substances subject to Authorisation (REACH Annex XIV) can only be used if the European Chemicals Agency (ECHA) gives specific dispensation to use it for specific purposes under controlled conditions, with the aim of encouraging substitution of SVHCs with safer alternatives.

The Authorisation process for SVHCs is complicated and lengthy, and comprises the following three stages:

  • Stage 1: Proposal of suspected SVHC by Member State authorities in a Community Rolling Action Plan (CoRAP). The current CoRAP list includes 86 suspected EDs. If the evaluation supports further regulatory action, the ED is placed on the Candidate List for inclusion in Annex XIV of REACH. This list currently includes: (a) six EDs, mainly phthalate substances, that affect human health; (b) twelve substances that affect aquatic organisms.
  • Stage 2: Draft recommendation by ECHA for the details of the Annex XIV entry, which includes the sunset date, after which the substance may not be used without Authorisation; the deadline for industry to apply for Authorisation if they wish to continue using the substance; and uses exempted from Authorisation. After a consultation period, allowing stakeholders to comment and authorities to review, the substance may be placed on REACH Annex XIV. Two ethoxylated phenols are currently listed in Annex XIV as EDs with estrogenic effects.
  • Stage 3: Applications for Authorisation: If industry applies successfully, it can get an authorisation to use a substance that is on Annex XIV. For each application, ECHA will launch an 8-week public consultation to identify possible alternatives to the substance for the particular use. If the applicant can demonstrate, using a socio-economic analysis, that continuing to use the substance is better than using alternatives, then the application is accepted.

Hazard Classification

Chemical suppliers are obliged to classify their products according to certain hazard criteria, as prescribed by the CLP Regulation [6]. EDs are classified in the same way as any other chemical.

Classifying EDs as CMRs is also may be necessary, as EDs frequently manifest themselves in terms of carcinogenicity and reproductive toxicity in mammals, which are assessed during the hazard classification process. However, some ED effects are outside the scope for classification, eg behavioural effects.

Also, for the environment, the classification criteria do not include endocrine disruptor effects, but rely on simple observations such as death or immobilisation.

These shortcomings in the current classification system have led some to propose a separate hazard class for EDs. This would allow development of specific testing to be included in the classification criteria, and human and wildlife effects could be dealt with in one class in a coherent fashion.

Changes to classification criteria by petitioning at UN level for the GHS, would take many years to accomplish.

Conclusions

The WHO/IPCS definition of an ED is very broad, and does not discriminate between EDs of high and low concern. EDs are recognised within REACH as being substances of very high concern. However, a disadvantage of this is that EDs of a lower hazard might go unregulated.

Many chemicals interact with the endocrine system, as determined by receptor binding assays and other in vitro testing, and so are potential EDs. Many other substances have been implicated as potential EDs by structure–activity relationships and computational methods. This high-throughput screening approach is prevalent in the US.

These methods are comparatively cheap, but positive results in vitro or in silico do not predict adverse effects in animals, and the assessor must use a weight of evidence approach until definitive, resource-intensive mammal and wildlife tests are performed. Testing for EDs also requires testing additional to that currently required by the REACH Regulation.

There is therefore legitimate concern that REACH is not achieving its stated aim of high protection for human health and the environment. The situation could be improved by updating current REACH testing methods to include ED measurements, or including specific ED testing in the REACH test battery.

It may be beneficial to form a separate regulatory classification for EDs, based on potency, specificity, severity, and irreversibility.


References

[1] Effects of TBT Contamination of the Sea; Fisheries Research Services. [back]

[2] Regulation (EC) No 1907/2006 of the European Parliament and of the Council of 18 December 2006 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH). [back]

[3] Endocrine Disruptor Screening Program Test Guidelines – OPPTS 890.1600: Uterotrophic Assay [EPA 740-C-09-0010]. [back]

[4] Endocrine Disruptor Screening Program Test Guidelines – OPPTS 890.1400: Hershberger Bioassay [EPA 740-C-09-008]. [back]

[5] Endocrine Disruptor Screening Program Test Guidelines – OPPTS 890.1100: Amphibian Metamorphosis (Frog) [EPA 740-C-09-002]. [back]

[6] Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures (as amended). [back]


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