Neurotoxicity is damage to the structure and/or function of the peripheral and central nervous systems. It is a common outcome of exposure to hundreds of environmental chemicals, which act via a wide range of mechanisms. Due to the fundamental importance of the nervous system to a fully functioning body, the neurotoxic effects of many chemicals have been well investigated.
There is evidence from a number of studies of a difference in susceptibility to environmental neurotoxins between genders. Males appear to be more vulnerable than females. There may be many reasons for this difference, a key one being the neuroprotective activities of the gonadal (sex) hormones, which differ between males and females. The female hormone, oestrogen, is thought to have greater protective activity, from a wide range of chemicals than the male hormone, testosterone.
This report will examine the available evidence of a gender difference in susceptibility to environmental neurotoxins, and look into the actions of hormones within the nervous system as one of the main reasons for this difference.
The nervous system (NS) is a fundamental component of a fully functioning human body. Due to the immense importance of the NS, any damage that occurs to this system will have huge repercussions throughout the whole body. Unfortunately, the NS is extremely vulnerable, and neurons, with their unique shape, and long, thin extensions protruding from their cell bodies, are highly susceptible to degeneration, from ageing and from exogenous substances (1, 2).
It has been observed that exposure to a range of different environmental chemicals can have adverse effects on the NS, resulting in degeneration of neurons, and leading to onset of various neurological diseases (2, 3). The developing NS in particular is extremely sensitive to the effects of such chemicals (2, 4). Prenatal, and early postnatal, exposure to environmental chemicals, such as lead and those in tobacco smoke, can affect the developmental process within the Central Nervous System (CNS). This can lead to slowed and incorrect development, and neurological problems in the early years of life (4).
From both animal studies, and human case reports of inadvertent exposures, there is also evidence to suggest a difference between males and females in their susceptibilities to neurotoxicity of some environmental chemicals (5). There are a number of reasons why this may be, including differences in amounts and activities of metabolic enzymes, differences in rates of absorption between the sexes, different rates of clearance of exogenous substances from the body, and differences in exposure to neurotoxic chemicals; diet, hobbies, occupations, etc (6). However, a key reason may be the neuroprotection that is conferred by gonadal hormones, and their metabolites, within the NS (5).
The aim of this report is to research evidence of sex differences in responses to environmental chemicals, and investigate hormonal influences as one of the reasons for this difference.
Neurotoxicity of Environmental Chemicals
Neurotoxicity is a term used to describe damage to the structure and/or function of the peripheral NS (PNS) and CNS, brought about by exposure to particular exogenous substances (7, 8), which act via a range of mechanisms to induce cellular changes, and often cell death (7). Neurotoxicity can be seen in all ages of individuals exposed to hazardous chemicals, however, the developing NS is particularly vulnerable to their effects (2, 4, 7). Development of the NS involves a series of very specific steps, over a prolonged time period, each one occurring only when the previous is finished, and disruption to these events leads to incorrect development and neurological problems (4). The blood-brain barrier (BBB), which prevents many substances from passing to the brain, is not fully complete until several months of age, leaving the NS susceptible to damage (7). The entire NS is not fully mature until puberty (4). A great number of the reports published concerning neurotoxic effects of chemicals have reported observations on child subjects. This is due to the fact that the developing NS is much more vulnerable, and so the neurotoxic effects may be more easily noticed.
There are over 200 chemicals that have been confirmed as neurotoxic to humans (and other animals)as a result of exposure to them (3). A number of these chemicals are identified in Panel 1 (3), and can be divided into groups; metals, organic solvents, pesticides, and other neurotoxic chemicals.
Panel 1. There are over 200 chemicals known to cause neurotoxicity in humans. This list identifies some common ones. Adapted from (3). Chemicals in bold and red are those identified within this report.
Different toxins have distinct mechanisms through which they influence the NS. This depends on dose, route and duration of exposure (9). Those chemicals which are most widespread in the environment, and those which cause the most drastic effects, have been extensively investigated, and many of the mechanisms causing neurotoxicity have been identified (9). Given the knowledge of these effects, it is important to investigate the possible neurotoxic influences of the large number of other chemicals prevalent in the environment.
Mechanisms of neurotoxicity
The main mechanisms encompassed by the afore-mentioned groups of substances include;
- induction of oxidative stress,
- alterations to neurotransmitter synthesis including inhibition of synaptic signalling,
- accumulation of the substance within mitochondria leading to dysfunction,
- alterations to the flow of ions across neuronal membranes,
- activation of second messengers to induce apoptosis or inhibit neurogenesis,
- disruption of DNA/RNA, affecting the differentiation and functioning of glial cells, to indirectly influence neuronal cells,
- alterations to membrane fluidity,
- abnormal expression of neurotrophic factors
There is a requirement for metals in many body processes, including within the NS, providing an additional mechanism by which exogenous metals can induce neurotoxicity (17). They can compete with essential metals for protein binding sites and influence cellular processes (17). For example, lead competes with zinc, which is known to have binding sites present in many important receptor channels, such as the N-methyl-D-aspartate (NMDA) receptor involved in glutamate signalling at the synapse. Lead can displace zinc, and therefore alter functioning of these channels, and so influence glutamatergic functions in the NS (13, 14, 17).
A relatively recently proposed mechanism thought to induce neurotoxicity via environmental chemicals, is endocrine disruption. Endocrine disruption is believed to be a crucial mechanism of most neurotoxicants, including metals, solvents, pesticides, Polychlorinated Biphenyls (PCBs), Diethylstilbesterol (DES), etc (21-25). Endocrine disrupting chemicals act by mimicking, enhancing, or antagonising the effects of endogenous oestrogens and androgens (21, 22). Their actions can result in alterations to hormone synthesis and/or release, altered transport and clearance of hormones, altered binding of hormones to their receptors (by binding themselves, thereby either mimicking hormone response, or blocking hormonal activation (24)), or altering components of pathways following receptor activation (22). An example of an endocrine disrupting mechanism is one used by lead, which lowers blood levels of testosterone, thereby de-masculinising certain areas of the male brain, and PCBs, which both mimic and antagonise various oestrogenic functions, and disturb production of androgens (21).
As hormones are known to have a role in the development of the CNS, including sexual differentiation (26), disruption to their activities may result in disruption to the development of some brain areas, and the possibility of feminisation or masculinisation of particular brain areas (21-25). The neuroprotective function of hormones (discussed later) may also be hindered due to the endocrine disrupting actions of certain chemicals, allowing for their other neurotoxic mechanisms to have greater damaging effects.
Carrying out investigations into the effects of neurotoxic chemicals is much more difficult in humans than it is in other animals, due to the greater difficulty in controlling the surrounding environment and its influences, and there are many potential variables that can have an effect on the overall result, in particular exposure to other environmental chemicals, drugs, alcohol, tobacco, education, culture, etc (27-31). All the potential confounding factors must be taken into consideration in order to analyse the neurotoxic effects only of the chemical in question (32). Often, environmental chemicals induce delayed neurotoxicity, whereby a patient does not present with symptoms until well after exposure to the chemical has ended, providing another problem to investigators (4).
There are many different symptoms that can present upon neurotoxicity; migraines or headaches, confusion, memory loss, Multiple Sclerosis (MS)-like symptoms, problems with sleep, balance and hearing, attention impairment and trouble concentrating, anxiety and depression (8). Alterations to cognitive function, motor function and behaviour are common outcomes of neurotoxicity, and are a useful assessment of the effects of exposure to chemicals (32, 33). There are a wide range of different tests commonly used to assess neurotoxicity to the PNS and CNS (4, 32, 33). Measurements of functions such as motor reflexes, insensitivity to pinpricks on the skin, or impairment of sensitivity to temperature and vibration, provide evidence of PNS toxicity (4, 32, 33). Other functional tests, including IQ (Intelligence Quotient) tests, memory tests, assessment of mood and personality, and behavioural questionnaires, are used to assess toxicity to the CNS (4, 32, 33).
Damage to the Nervous System can also be established by use of various brain imaging techniques (e.g. Computed Tomography, Magnetic Resonance Imaging) (9). These are useful in observing physical alterations to brain size and appearance caused by brain tissue atrophy following neurotoxic exposure (9). It is also possible, using these images, to ascertain which regions of the brain are particularly affected (9, 33-35).
Despite the large quantity of literature outlining investigations concerning exposure to different neurotoxic chemicals, there are relatively few publications available that have identified a difference in response between males and females. Differences between susceptibilities of a range of age groups, and groups with varying levels of exposure, have been acknowledged frequently (27, 36-38), however reports are rare in which results for men and women are assessed independently, therefore it is often difficult to determine any differences in susceptibility between the sexes.
Many reports record numbers of each sex taking part in the study, and match controls accordingly, then proceed to analyse results as a whole (27, 28, 39-45). Others exclude female subjects altogether, rather than including analysis of female results, but separate from the male (29, 30, 46-51). This is often the case when the number of female subjects is small compared to men. However, the results could still be analysed, and any differences between them could be noted. Some fail to establish which sexes have been used at all (52-54).
Nevertheless, there is evidence from a number of reports, of a difference between genders in neurological functioning following exposure to neurotoxic chemicals. An extensive search using MEDLINE and EMBASE, of published studies and case reports into neurotoxicity of environmental chemicals, identified a number of studies which observed differences between males and females. For the purpose of this report, only those chemicals with gender differences have been mentioned.
Evidence of Gender Differences in neurological outcomes of exposure to Neurotoxic Chemicals
There are roughly 40 different metals that exist in the environment, some of which are essential for life to occur (e.g. copper, zinc, etc), others which aren’t (e.g. mercury, lead, etc) (9). Exposure to metals in the environment has been known to cause adverse effects to both the adult and child human NS for many years (3). The neurotoxic effects of these metals are particularly well characterised, and have been well investigated. Included in this report are three of the major neurotoxic metals, of which there has been much exposure to in the environment, and of which there has been some indication of a sex difference in susceptibility to neurotoxic effects; mercury, lead and manganese. These three metals have been more extensively investigated than others, and therefore sex differences observed should not be ruled out of others, and may also be noted if they are as well examined.
Mercury can take various different forms, each of which has distinct effects on human systems (18). Methylmercury (e.g. contaminated seafood), ethylmercury (e.g. Thimerosol, a component of some vaccines), elemental Mercury (present in industrial vapours), and inorganic mercury compounds (e.g. skin lightening creams) (18). Of these forms, methylmercury has been acknowledged as having the greatest detrimental effect on the correct functioning of the human NS, and in particular, the developing nervous system of children (18). In adults, methylmercury is thought to damage specific brain regions, such as the visual cortex, and parts of the cerebellum, whereas in children, as the NS is not completely developed, the effects are thought to be more widespread (7).
It has been observed in a number of studies that male children show greater impairments in NS functioning following exposure than female children. In certain neurological tests, which have an association with methylmercury exposure, namely those assessing finger tapping, tendon reflexes, and leg coordination ability, males achieve poorer results (8, 36, 37, 55-57).
As the majority of studies reporting results individually for male and female subjects are those carried out in children, the main sex differences reported here have been observed in children. However, similar results are noted in those adult investigations where males and females were analysed separately (27).
McKeowyn-Eyssen et al. (1983), Cordier et al. (2002), Myers et al. (2003), Grandjean et al. (1998), and Marsh et al. (1987), all carried out numerous different tests on school children exposed to methylmercury at varying concentrations, pre- and post-natally. Each of these groups identified that, for those tests which have been shown to be more affected by increasing methylmercury levels, including finger-tapping, abnormal muscle tone, tendon reflexes, and leg coordination, male children showed poorer results (19, 57-60). McKeowyn-Eyssen et al. (1983) carried out the same tests on adults, and found an indication of a similar sex difference, with men being more likely than women to develop neurological disorders, following increases in methylmercury levels (37).
Davidson et al. (2000) found that male, but not female, responses in neurological tests increased with methylmercury exposure, which is the opposite of the expected results, however, numerous unexamined variables were identified, which could have had influences on the results of the tests (31).
Holmes et al. (2003) identified a link between mercury exposure and autism in children. Higher mercury levels in the hair were found to be associated with milder autistic symptoms (61). Perhaps because those children with milder symptoms were more able to excrete the mercury through their hair, before too much damage occurred. There was a greater number of females showing milder autistic symptoms, and a greater number of males showing severe autistic symptoms (61).
From the evidence put forward here, there is a definite implication of a greater susceptibility for males than females to the neurotoxic effects of methylmercury exposure. There is an increased risk of neurotoxicity for children of women with increased levels of mercury in the hair (61). Hair mercury levels in subjects themselves, following equal exposure between the sexes, has been observed on numerous occasions as being lower in males than females, when associated with neurological problems (37, 61, 62). It may be that females have a better ability to excrete mercury through the hair than males, so less is present in body tissues.
Lead has long been known as a neurotoxicant, and its widespread release into the environment over the years has resulted in many neurological problems, mainly linked to learning difficulties (17), that have been well studied and characterised (3).
Lead toxicity is thought to occur mainly in the hippocampus, cerebellum, and prefrontal cerebral cortex and again, it is thought that children, with their NS still developing, are at greatest risk to the neurotoxic insults of lead (7), so the majority of reports found here have been carried out in children.
The elimination of lead from many environmental sources, such as motor vehicle petrol, and paints, has seen a decline in the amount of toxic lead exposure (7). However, it is still a problem in many areas, for example those homes where lead paint has been used in decoration (17).
There are a number of studies that have reported a difference in cognitive impairments between male and female children. Tests carried out on school children, in South America, the UK and USA (38, 63-66), all identify a larger correlation between lead levels in the blood and poor cognitive ability in males than in females, while Wasserman et al. (1998) state that mothers reported behavioural problems with male children exposed to lead, more often than with exposed female children (67).
An assessment of behavioural problems associated with lead exposure in American children (68) and an assessment of intelligence of children following lead exposure in Port Pirie (69), identified no difference between males and females in the results of their tests, while an assessment of the capabilities of children in school, and association with lead exposure (70), along with another investigation of child IQ by Needleman et al. (71), observed results to suggest females were more susceptible to lead neurotoxicity than male subjects, as they appeared to have greater prevalence of learning difficulties associated with lead.
So, there appears to be a significant amount of evidence implying a gender difference in neurotoxicity associated with lead exposure. The majority of reports imply an increased susceptibility for males; however it is important for groups to look at sex differences in future studies, in order to ascertain conclusive results.
This evidence also provides a need for investigation of sex differences in effects of lead exposure in adults.
Manganese is another commonly used metal that can cause a toxic effect the NS upon exposure (20, 29, 40, 46, 47). There is a risk of manganese toxicity in various professions, in particular, welding (29, 46), but also through drinking or washing in water containing extraordinarily high levels of manganese (20, 40). There are a large number of reports confirming the neurotoxicity of manganese (20). Investigations have shown decreased intellectual ability in children over-exposed to manganese (40), and mood disturbances in men exposed occupationally (e.g. welders, factory workers.) (29, 40, 46, 47).
In children, a report into an association between hair manganese levels and prevalence of hyperactivity, found that while there was a higher amount of manganese present in girls than boys, no difference was found between the sexes in assessment of neurological behaviour tests (72). Perhaps female brains are better able to cope with a higher amount of manganese.
In adults, Dietz et al. (2001) found that a relationship between levels of manganese exposure and its effect on the Globus Pallidus area of the brain was seen only in men. These investigators give the reason that female workers have lower blood concentrations of manganese, and have a lower cumulative exposure index (73). However, they do not state whether there was a difference in actual exposure between sexes. If the exposure levels were the same, this could be an indication of increased susceptibility to males. In another study, results of neurological tests following manganese exposure were poorer for men than for women (74).
As the majority of studies on manganese actually exclude females from results, or do not give separate results for each sex, it is difficult to make any definite assumptions about gender differences in neurotoxicity susceptibility. Implications from the three studies above provide a suggestion of a sex differences in manganese toxicity, with a greater effect within males. However, in future studies, where possible, females should be included, and the results analysed separately, in order to establish conclusive evidence for sex differences in neurotoxicity to manganese.
There is a vast array of solvents that are used in many different industries and work places, meaning daily exposure for many different workers, including hairdressers, laboratory workers, painters, dry cleaners, and carpet layers, among others (33, 75-78).
Due to the composition of solvents, they are particularly dangerous to the tissues of the NS. They are lipophillic compounds, and therefore have strong affinity for tissues rich in lipids, including the brain (33, 79). It is thought that psychomotor performance is the most common deficit (51) of solvent exposure, and prolonged exposure can cause permanent damage (15). Other symptoms include anxiety, insomnia, irritability, memory loss, fatigue and seizures (15, 33, 75). Solvent substances most often consist of a mixture of different chemicals, which can affect different regions of the brain. This can result in difficulties determining the toxic effects of a particular chemical (9).
There have been many studies published that report clear association between solvent exposure and neurological deficits. Nelson et al. (1994) report that solvent exposure in workers at an automobile assembly plant, correlates with increased neurological disease, and, noticed in particular, an association with increased prevalence of a condition closely resembling MS (52). Cavalleri et al. (1994) obtained results to indicate deterioration of colour vision in factory workers following perchloroethylene exposure, even at low levels (53), and Boor et al. (1977) confirm a damaging effect of toluene on the CNS (54), a chemical that is also known to effect CNS development prenatally (3).
Alcohol (Ethanol) is a major environmental solvent, although exposure rarely occurs occupationally, and it is most often taken in voluntarily (3). Hommer et al. (2001) studied the brain volumes of alcoholic and non-alcoholic men and women, and found that alcoholics had a much smaller volume of grey matter than non-alcoholics. This difference was found to be much more significant in females than males, suggesting an increased susceptibility of females to neurotoxic effects of alcoholism (34). In contrast, Pfefferbaum et al. (2001), in the same journal publication, indicated that the results of their study into alcohol effects on brain structure, show larger cortical sulci and lateral and third ventricles found in the alcoholics compared to non-alcoholics, which was a much greater and more significant difference in male subjects than female subjects. They also note that female brains show quicker and more effective recovery than those of males during abstinence (35). Jacobson (1986) carried out a study examining the brains of male and female alcoholics compared to non-alcoholic controls. It was noticed that the appearance of the brains on a CT scan was different between alcoholics and controls. Also observed was the fact that females appear more susceptible to structural changes in the brain following chronic alcohol intake, but are much more effective at recovering following cessation of intake, and the recovery occurs much quicker (80). Taking these 3 reports into consideration, there may be a difference in susceptibility of particular brain areas in males and females; however, females consistently recover more quickly from damage than males, indicating perhaps, a decreased susceptibility to long term damage.
Neurophysiological deficits have also been reported in numerous studies of children exposed to alcohol pre-natally (81-83). However, few have noted results separately for male and female children. Nanson and Hiscock (1990) observed that female Fetal Alcohol Syndrome (FAS) children appear to have a higher IQ than males with FAS (83).
As mentioned above, the majority of studies into other solvents, such as toluene, trichloroethene, n-hexane, chlorinated solvents (84), and solvent mixtures (49, 50, 76, 78, 85) in the workplace, report an obvious detrimental effect on the CNS, PNS, or both, following exposure. However, the majority included only men in the reports, or male and female results were analysed together.
Again, it has been observed that the developing NS is especially susceptible to the neurotoxic effects of solvents, due to their high affinities for the brain’s lipid tissues (33, 79), and the BBB not being fully formed (7).
Laslo-Baker et al. (2004) and Till et al. (2001) carried out studies on organic solvent exposure in pregnant women, taken in accidentally from occupational exposure, and the effects on neurodevelopment of their offspring. Both groups confirmed that children exposed pre-natally had poorer cognitive functioning than those not exposed, with lower results in neurological tests (75, 86). Again, no distinction was made between results for female and male children.
Considering the obvious effects of solvents, including alcohol and toluene, on the NS, and the observations of sex differences from other neurotoxins, and the implications of sex differences in effects of alcohol mentioned here, it should be suggested that future studies automatically investigate male and female results separately, and allow for observation of any differences in results.
The term pesticides encompasses a wide range of chemicals, commonly used within a wide range of industries, particularly agriculture (87, 88). Included are the sub-groups; organophosphates, organochlorines, fumigants, and herbicides, all of which act to damage the NS of an organism, either directly, or via alteration of the cellular mechanisms that support it (87). Pesticides cause concern for human health as they are extremely widely used, and so readily released into the environment (88). It has been known for a long time that exposure to certain levels of these chemicals will adversely affect the human NS, as well as those organisms they are designed against (87, 88). Indeed, numerous studies have linked exposure to various pesticides with a number of neurological disorders, including Parkinson’s disease (87, 89).
In a similar situation to that for metals and solvents, there are many publications from groups investigating the effects of pesticide exposure on the human Nervous System, using an array of cognitive and neurobehavioural tests, with almost every study confirming the presence of some form of Neurotoxicity in subjects exposed to a range of doses.
The following reports have identified separate results for neurological effects of pesticide exposure on male and female subjects, and an apparent greater effect on males.
A report investigating the influence on the onset of Parkinson’s and Alzheimer’s Diseases in elderly people living in the south of France, where pesticides are used daily in vineyards, noted a significant association between these disorders and pesticide exposure, in males only (90), suggesting a potentially increased susceptibility to males.
Stallones et al. (2002) acknowledge males being at increased risk of developing neurological problems related to pesticide exposure than females, in an investigation into farmers, and their families in Colorado, USA (91), with the percentage of illnesses caused by exposure to pesticides almost three times greater in males. An assessment of neurobehavioural activity of Hispanic agricultural workers (92) identified a significant difference between the genders on results for 2 out of 10 tests, with females scoring lower than males. In the remaining tests, no significant differences were found between the sexes, although all exposed subjects faired worse than control, non-exposed (92). Similarly, pesticide-exposed Ecuadorians achieved lower outcomes in neurobehavioural tasks set by Cole et al. than did non-rural, unexposed Ecuadorians, and females were found to respond better in one task, with no significant difference between genders in others (93, 94).
Guillette et al. (1998), carried out an assessment of Preschool children in Mexico, exposed to pesticides through living in close proximity of farm land. They identified a significant difference between those exposed and those living further away from the farm lands, with females performing better than males in several of the neurological tests (95).
It appears that when there is a gender difference observed in the neurotoxic effects of pesticides, females tend to fair better than males, implying an increased susceptibility of males to the influences of pesticides on the NS. As it is more commonly males that are in the closest proximity to pesticides, within farming industries in particular, this could have some influence on this hypothesis. However, as the differences are also apparent in male and female children, with equal exposure, it does indicate a greater risk for males.
The finding that there was only a significant difference in some tests may indicate an increased susceptibility of some brain areas in males over others, which correlates with results of studies of alcohol and tobacco smoke (below).
Other Sources of Environmental Neurotoxicity
The chemicals contained in tobacco smoke, particularly nicotine, are now known to cause a variety of neurological problems, in addition to their other effects, including behavioural and cognitive problems during development, tremor, and an increased risk of stroke, from both smoking directly, and through passive smoke; inhalation or exposure prenatally (96-100). Various groups investigating toxicity caused by intake of tobacco smoke have described minor sex difference in the neurological outcome.
Louis (2007) reports that, when looking into hand tremor as an outcome of tobacco smoking, the difference in score between smokers and non-smokers is greater in women than in men, which would indicate more of a susceptibility to women, rather than men (96).
Jacobsen et al. (2007) investigated auditory and visual attention in adolescent smokers and non-smokers, with and without prenatal exposure to tobacco (101). They observed that different areas of the brain are apparently affected differently in male and female subjects exposed to tobacco smoke. In females, both auditory and visual attentions appear equally vulnerable, performing slightly more poorly in visual tests than males, while in males, auditory attention seems significantly more affected than visual attention, and in this auditory test, males performed substantially worse than females (101). The results of this investigation, put together with those from the Louis (2007) report, point towards sex-specific variation