Effects of hazardous chemicals

Effects of hazardous chemicals

Published on 12 Jan 2023

Since the 1940s, society has seen increasing rates of many diseases and health problems, including various cancers, autoimmune disorders, behavioural and attention deficit disorders, male infertility, premature puberty, and an explosion in rates of obesity and cases of diabetes.

Obviously, lifestyles have changed a great deal in the last 70 years, but so has the presence of chemicals in the environment, food and consumer goods. More than ever before, people are exposed to complex mixtures of man-made chemicals in food packaging materials, air pollution, furniture dust, electronic waste, toys and so on. Measurable levels of hundreds of man-made chemicals are routinely found in people, regardless of age or where they live.

Determining the health effects of these complex, low exposures is highly challenging to environmental health researchers: the technology to detect chemicals in people is in some cases barely a decade old, and the science for determining the effects of chemicals on health is still very much under development.

Nonetheless, as research gathers pace, evidence is mounting that everyday exposure to chemicals could be playing a significant role in the onset of health problems. Although the precise nature of these effects is not yet clear, the overall implications for health mean it is very important that exposure to potentially harmful substances is limited as much as possible.

According to the UNEP report “Global Chemicals Outlook” from September 2012 poisonings from industrial and agricultural chemicals contribute to more than a million deaths every year worldwide. This figure is among the top five leading causes of death globally, after HIV/AIDS, tuberculosis, road traffic accidents and malaria.

Carcinogens, mutagens and reprotoxins

In Europe, chemicals are classified and their use regulated according to the type of harm they can do. A particularly significant category is chemicals which are carcinogenic, mutagenic or reprotoxic (CMRs).

Carcinogens cause cancer, either by directly damaging the DNA or harming the body’s defence mechanisms against the disease. Mutagens cause changes to the DNA; since DNA damage initiates many cancers, mutagens are considered highly likely to be carcinogens. Reprotoxic substances are harmful to sexual function and development of the unborn child.

CMRs are considered to be high-risk substances. However, of the approximately 80,000 chemicals on the market, only 3,000 have undergone any substantial degree of toxicity testing, resulting in 800 being identified as CMRs. Which other chemicals should be classified as CMRs is a matter of ongoing debate and research.

Besides being a CMR, other ways for a chemical to be toxic include potential to harm the brain and nervous system (neurotoxins); ability to cause diabetes, immune and autoimmune disorders (immunotoxins), cardiovascular and respiratory disorders, obesity (obesogens) and interfere with hormone signaling (endocrine disruptors, such as those which interfere with thyroid function), ability to cause allergy (sensitizing) to name but a few.

Evidence is mounting that everyday exposure to chemicals could be playing a significant role in the onset of health problems

Windows of development

“Windows of development” concerns how the health effect of chemicals can differ depending not only on the degree of exposure but also on the timing of the exposure, with the same chemical having different effects at different times.

The body seems to be uniquely sensitive to effects when organs in the body are developing and being programmed for healthy functioning, such that if chemical exposure occurs in this period, it can cause permanent alterations in physical function, setting a person up for life-long consequences from a short period of exposure. This damage, although already caused, may not be apparent until decades later.

Endocrine disruption

Endocrine disruptors interfere with hormone signaling in the body. Because of the vital role hormones play in all physical processes in the body, including healthy development of the foetus, the growth of cancers and regulation of emotions and behaviour, EDCs are thought to play an important role in mediating many of the health problems which are becoming more common today.


Cancer is uncontrolled, invasive cellular reproduction

Cancer is a family of around 200 diseases which begin when cells reproduce in an uncontrolled manner, invading and destroying healthy tissue. Chemicals can be carcinogenic by directly damaging DNA, increasing the risk of genetic mutations by altering cellular reproduction cycles, or by interfering with the body’s various fail-safe mechanisms which stop cancers developing.

Chemicals and cancer

The European Union CLP regulation (Classification, Labelling and Packaging of substances and mixtures) has identified 904 substances and substance groups that may cause cancer (R45 or R49), according to the European Commission ESIS database.

The International Agency for Research on Cancer (IARC) classifies 165 agents, many of which are chemicals, as either “definitely” or “probably” carcinogenic to humans, including benzene, vinyl chloride, chromium VI compounds, dioxin and formaldehyde (IARC 2010).

Endocrine disruptors and cancer

There is a growing body of evidence that endocrine disrupting chemicals (EDCs) may increase the risk of cancer by interfering with hormonal signalling in ways which make cancers more likely to initiate or grow (Diamanti-Kandarakis et al. 2009).

Exposure to EDCs during developmental periods can certainly have life-long implications for cancer risk, as was proven by the discovery that the daughters of women who took the oestrogenic drug diethylstilbestrol (DES) during pregnancy had a greatly increased risk of developing rare reproductive cancers (Herbst et al. 1971).

For chemicals we encounter in our everyday environment, studies of female mice exposed in the womb to Bisphenol A (BPA) have found they are more likely, when adults, to develop abnormal breast tissue which is more likely to become cancerous (Vandenberg et al. 2007; Murray et al. 2007).

Males can also be affected by oestrogenic compounds, with rat studies finding that BPA exposure after birth can cause an increase in pre-cancerous prostate growths in adulthood (Prins et al. 2011).

Given that prostate cancer and breast cancer are the most common cancers in men and women respectively, widespread exposure to other oestrogen-like compounds such as parabens and alkylphenols is a matter of concern.

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What is obesity?

Obesity is the excess accumulation of body fat, to the degree that it becomes harmful to health. 26 percent of men and women in the UK are obese. In the US, 32 per cent of men and 35 per cent of women are obese. By 2030, it is expected that 50 per cent of US and 40 per cent of UK adults will be obese. By this point obesity will have overtaken smoking as the biggest preventable cause of chronic, non-communicable diseases (Wang et al. 2011).

What are obesogens?

Obesogens are chemicals which affect the body’s ability to control its weight. We know that diabetes drugs cause obesity, and researchers have also discovered that the children of mothers who smoke while pregnant are much more likely to become obese (Monasta et al. 2010).

We do not really understand how chemicals can cause obesity. However, experiments have shown that the antifouling agent tributyl tin causes rats to produce more fat cells than usual (Kirchner et al. 2010). Other experiments have found that the oestrogenic drug diethylstilboestrol (DES) makes a mouse’s metabolism act as if it is in a low-food environment, so the mouse becomes fat if food is in fact plentiful (Newbold et al. 2009).

Which chemicals are obesogens?

Very few chemicals have been tested to find out if they are obesogens. Studies have found that the offspring of pregnant rats exposed to small quantities of bisphenol A (BPA) gain more weight than is normal, especially when on a diet high in fat (Wei et al. 2011). Similar effects have been found for organophosphate insecticides (Slotkin 2011).

Epidemiological studies have found that people exposed to persistent organic pollutants (POPs) tend to have more body fat than normal (Lee et al 2011). Other chemicals which may be obesogenic include non-stick PFOAs, phthalates, nonylphenol, dioxins and furans, small particulates in air pollution and some brominated flame retardants.


What is diabetes?

Diabetes is a group of metabolic diseases in which a person’s blood sugar levels are too high, caused by failure of the body to either produce insulin (type-1 diabetes) or respond to it (type-2 diabetes). It is becoming increasingly common, with 11 per cent of Americans over 20 years of age now estimated to have the disease.

Type-1 diabetes normally develops in childhood as the result of an autoimmune disorder where the body destroys its own insulin-producing cells. Type-2 diabetes usually develops in adults, although the age at which it develops is coming down (Koopman et al. 2005).

How are chemicals linked to diabetes?

The rise in incidence of type-2 diabetes is generally attributed to an aging population and changes in modern lifestyle such as reduced physical activity and eating more foods high in animal fats. However, researchers are beginning to find consistent correlations between increased risk of type-2 diabetes and exposure to persistent organic pollutants (POPs) such as DDT, PCBs and dioxins (Carpenter 2008, Lee et al. 2010).

Because diabetes is an autoimmune disease, can be brought on by obesity, and is a disorder of insulin resistance, then any chemicals which contribute in some way to increased risk of any of these problems may also be contributing in some way to increasing rates of diabetes. Animal studies are showing links between a range of chemicals in the environment and the development of these problems.

Which chemicals are a problem?

Contaminants associated with diabetes in humans include some air pollutants, nitrate/nitrite/N-nitroso compounds, bisphenol A, cadmium, some pesticides, some persistent organic pollutants (including dioxin and PCBs), some flame retardants, selenium and phthalates (Thayer et al 2012).

Some air pollutants, bisphenol A, some persistent organic pollutants (including PCBs) and some pesticides have been found to promote insulin resistance in animals. Air pollutants, bisphenol A, some persistent organic pollutants (including PCBs), some phthalates, some heavy metals, some pesticides and solvents have been found to increase weight gain in animals.

Since many of these chemicals, especially PCBs, flame retardants and dioxins, are produced in electronics waste streams, reducing the use of chlorinated plastics and halogenated compounds is important in electronic goods.

The thyroid function

What is the thyroid system?

Thyroid hormones perform a highly varied number of functions in many of the body’s organs and are an essential part of the body’s metabolic control system, regulating how quickly the body uses energy and governing the body’s sensitivity to other hormone signals.

One reason the possibility of chemical interference with the thyroid system is of particular interest to environmental health researchers is the key role thyroid hormones play in the development of the brain and nervous system, controlling synapse development, the growth of the myelin coating which sheathes nerve cells (essential for passing nerve signals), and proper functional arrangement of neurones in the brain.

How can disrupting the thyroid system affect health?

The mother provides all the thyroid hormone necessary for foetal development until relatively late in pregnancy. Even relatively minor fluctuations in levels of thyroid hormone in the mother’s blood during pregnancy can result in neurodevelopmental deficiencies in the foetus (Boas et al. 2006).

Children of mothers with low circulating thyroid hormone have been shown to have trouble with motor coordination, balance and other psychomotor skills. Attention deficit disorders and higher-than-average incidence of difficulties with spatial relations, perception, memory and language have been similarly linked.

Long-term low thyroid function in adults causes poor blood circulation. This reduces delivery of oxygen to body tissues and is associated with heart disease and cancer. Increased weight, depression, loss of hair and impaired cognitive function are other symptoms of low levels of thyroid hormones. There is evidence of generally reduced thyroid function in workers on primitive e-waste sites in China, thought to be due to common use of thyroid disruptors in electronic goods (Zhang et al. 2010).

Which chemicals can interfere with the thyroid system?

The thyroid system is complex and there are many ways in which chemicals can interfere with it, including inhibiting the thyroid gland’s ability to absorb iodine (phthalates and perchlorate), obstructing the enzymes which manufacture thyroid hormones (several pesticides and fungicides), and preventing thyroid hormones being activated or deactivated (lead, chromium, UV-blockers and the pesticide methoxychlor) (Patrick 2009).

Chemicals which are relatively well-researched with regard to thyroid-disrupting effects include PCBs, BPA, perchlorate, dioxins, pentachlorophenol (a breakdown product in the body of the pesticide hexachlorobenzene), triclosan and the PBDE flame retardants. Animal evidence of thyroid disruption exists for the phthalates DEHP, DnOP, DIDP, DNHP, DBP, resorcinol used in antiseptic creams and hair dyes and the flame retardant TBBPA, now the most commonly-used flame retardant in Europe (Zoeller 2010).


What is neurotoxicity?

Neurotoxins are substances which alter the function of the nervous system by damaging brain cells or the nerves which carry signals around the body. Some researchers regard substances which cause temporary changes to the function of the nervous system as also being neurotoxic.

Disorders associated with exposure to neurotoxic substances include impaired intelligence, impaired regulation of emotional responses, behavioural problems including attention deficit and hyperactivity disorders, depression, anxiety, memory formation, impaired physical coordination and increased risk of neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases.

How can chemicals harm the nervous system?

The healthy development and long-term function of the nervous system is governed by a wide range of physiological factors. Chemicals can have neurotoxic effects by directly and indirectly interfering with these processes.

For example, in the developing foetus some chemicals may prevent brain cells forming enough contact points with each other to communicate effectively. Other chemicals can impair IQ and cause behavioural problems by altering levels of hormones which are vital for the correct development of the foetal nervous system. In this respect, endocrine disruptors which target the thyroid system are of particular concern to researchers.

As adults, chemicals and pollutants can accelerate the rate at which adult nerve and brain cells degrade and die by increasing oxidative stress in the body. This is thought to increase the risk of Parkinson’s and Alzheimer’s diseases, bipolar disorders and chronic fatigue syndrome.

Which chemicals are causing problems?

At least 200 chemicals have been identified as potentially neurotoxic in humans and over 1,000 have been shown to be neurotoxic in animals, including:

  • PBDE flame retardants, associated with poorer concentration and lower scores in a range of childhood developmental and behavioural tests (Herbstman et al. 2010);
  • Heavy metals such as lead and manganese, which have been shown to impair intelligence (e.g. Ciesielski et al. 2012);
  • Phthalates, which impair physical coordination and reduce scores in a range of behavioural tests, with boys in particular affected (e.g. Whyatt et al. 2012);
  • Exposure to air pollution, which accelerates cognitive decline in adults (Weuve et al. 2012);
  • A range of pesticides, which increase risk of Parkinson’s disease and impair early mental development, affecting working memory, intelligence and perceptual reasoning (e.g. Wang et al. 2011);
  • BPA, which may affect regulation of behaviour and emotion (particularly in girls) and may feminise the behaviour of male boys (Braun et al. 2011);
  • Perfluorinated (non-stick and stain-resistant compounds), which may increase risk of attention disorders and impulsive behaviours (Hoffman et al. 2010)

Female reproductive health

What is female reproductive health?

In the context of chemicals and health, female reproductive health is a broad concept encompassing development of the female reproductive organs, sexual development, fertility, and the ability to give birth to healthy babies.

Issues include the age at which girls go through puberty, the age of menopause, the ease with which women can conceive, the chance of complications during pregnancy, the likelihood of giving birth prematurely or to an underweight baby, the healthy development of the foetus, and also diseases specific to the female reproductive system, such as endometriosis, polycystic ovary syndrome (PCOS) or ovarian, uterine, cervical and vaginal cancers.

How can chemicals affect female reproductive health?

Exposure to oestrogen-like chemicals is of particular interest to researchers, due to the hormone’s central role in a woman’s physiology. Clues as to the sorts of reproductive problems that might result from exposure to oestrogen-like chemicals before birth are coming from studying women who took the oestrogenic drug diethylstilbestrol (DES) while pregnant.

Between the 1940s and 1970s DES was prescribed to millions of women to prevent pregnancy complications, but was found to increase their daughters’ risk of developing breast cancer, precancerous lesions in the cervix, infertility and early menopause. Their daughters were also more likely to experience complications during pregnancy, with increased risk of preterm delivery, miscarriage, tubal pregnancy, stillbirth and preeclampsia (Swan 2000).

Subtle influences on reproductive health problems can also have broader health implications. For example, going through puberty early not only increases the risk of psychological problems, it also increases a woman’s lifetime exposure to oestrogen, which is a risk factor in breast cancer. Preterm birth and decreased birth weight increase the risk of a range of developmental and health problems for the newborn child, while early menopause increases a woman’s risk of osteoporosis, colon and ovarian cancer, heart attack and stroke.

To make things even more complicated, female reproductive health is not only determined by the function of oestrogen and other female hormones, but by many physiological processes in the body. For example, thyroid hormones play an important role in sexual maturation. The ways in which chemicals may affect reproductive health is therefore highly complex, particularly when healthy development of the foetus is considered.

How might chemicals be affecting female reproductive health?

The balance of sex hormones during development is critical so oestrogen-like chemicals are of obvious concern including phytoestrogens, some phthalates, BPA, parabens, phenols and UV filters in sunscreens, especially in light of recent research showing that a single exposure to estradiol or testosterone in newborn mice increases their risk of polycystic ovarian syndrome (PCOS) as adults (Sotomayor-Zárate et al. 2011).

Bisphenol A (BPA) is one of the chemicals most well-studied for reproductive health effects and has been found to change the development of the uterus in female primates exposed while in the womb. Possible links are also emerging between higher levels of BPA exposure and reduced numbers of viable eggs in women(Bloom et al. 2011).

The perfluorinated compounds PFOS and PFOA (also known as C8 compounds) have been linked to both delayed puberty in girls (Lopez-Espinosa et al. 2011) and premature menopause in women (Knox et al. 2011), while there is some evidence connecting phthalates, BPA and phenols with early puberty.

There is evidence that phthalates and pesticides have an effect on fertility and pregnancy outcomes, increasing the amount of time it takes a woman to conceive and increasing the risk of preterm birth and decreased birth weight (Burdorf et al. 2011).

PBDE flame retardants may also lower the chance of conception in a given time window (Harley et al. 2010), while phthalates may also increase the risk of endometriosis and uterine fibroids. Animal studies indicate that exposure to dioxin in the womb appears not only to make it more difficult for female offspring to become pregnant, but even the daughters and granddaughters of those offspring in turn (Bruner-Tran & Osteen 2011).

Male reproductive health

What is male reproductive health?

Male reproductive health refers to a man’s sexual function and fertility. It is a broad concept which includes physical formation of the genitals, sperm count, erectile function, age at puberty and risk of testicular and other male reproductive cancers.

A number of male reproductive health disorders are increasing, including malformations of the penis, low sperm count, testicular cancer (which has roughly doubled since the 1970s) and undescended testicle. The rise has been so rapid and so recent that the environment has to be a causal factor, and there is growing evidence from laboratory research that chemicals are contributing to the problem (Sharpe & Skakkebaek 2008).

How might chemicals affect male reproductive health?

Male reproductive health is complex and there are many ways in which it can be affected by chemical exposure. One area of particular concern is how chemicals can either suppress production or block the action of the hormone testosterone. If this happens while the foetus is developing, it can have a number of serious consequences for later development of the male genitalia. These include an increased the risk of the testicles not descending properly, of the penis not developing properly (a condition known as hypospadias), of low sperm count as an adult, and increased risk of testicular cancer.

Chemicals which have been shown to potentially have these effects include dioxin, organochlorine pesticides, some sun screens, some phthalates, herbicides such as linuron and diuron, vinclozolin and other fungicides such as chlorpyrifos, PBDE flame retardants and PCBs. None of these on their own have a particularly dramatic effect; however, people are generally exposed to combinations of many of these and, since the chemicals all affect the same biological system, it is possible these individual chemicals have significant combined toxicity.

A number of oestrogenic chemicals have been connected with feminizing effects in the male, including BPA, phenols, and the phthalates DINP and DCHP. PCBs, PFCs (in particular PFOS and PFOA) have been implicated in delayed puberty in boys. Dioxin, BPA and phthalate exposure is suspected of reducing sperm count in adults, while exposure to organochlorines and butyl parabens may damage the genetic material in sperm. The antimicrobial agents triclosan and triclocarban, DDT, parabens and some phthalates have been shown to affect testosterone production, with DEHP having a particularly powerful suppressive effect during later stages of foetal development.

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