In 1998 we reviewed the major activities in Europe and Japan on health issues
related to chemical mixtures (1). Although this survey was not exhaustive
and thus most probably incomplete, we were surprised by the large number of
research groups involved in the toxicology and risk assessment of mixtures.
The research programs varied from defining basic concepts in mixture toxicology
to straightforward toxicity and carcinogenicity testing of complex chemical
mixtures. The survey also revealed a growing interest among toxicologists and
regulators in the toxicology and risk assessment of chemical mixtures, with
the realization that simultaneous or sequential exposure to large numbers of
chemicals is a potential health concern. In this respect it is interesting to
note that already in 1985 the Health Council of the Netherlands included in
one of its advisory reports to the Dutch government a chapter on the potential
health consequences of combined exposure to chemicals, formulating some basic
rules on how to deal with combined exposures in standard setting (2).
Methods for dose addition and response addition were recommended.
In the United States, research programs on the toxicology of chemical mixtures
have existed for decades, and more than 15 years ago the U.S. Environmental
Protection Agency (U.S. EPA) published Guidelines for the Health Risk Assessment
of Chemical Mixtures (3), followed some years later by the Technical
Support Document on Health Risk Assessment of Chemical Mixtures (4).
The research programs varied widely and included studies on basic issues in
mixture toxicology such as physiologically based pharmacokinetic/pharmacodynamic
modeling of mixtures, use of physicochemical concepts for elucidating toxicological
interactions, development of statistical designs for experimental studies of
mixtures, and mathematical modeling of the processes involved in the carcinogenicity
of mixtures of carcinogens (5-8). Moreover, thousands of mutagenicity
studies and tens of carcinogenicity studies have been performed on (fractions
of) real-world mixtures such as diesel engine emissions, recycled drinking water,
urban air samples, tobacco smoke, foundry fumes, and incinerator emissions (9-33).
The passage of the Food Quality Protection Act and the Safe Drinking Water
Act Amendments in 1996 (34) has further raised awareness of chemical
mixtures health issues and has resulted in research on health risks associated
with multiple pathways of exposure and chemical mixtures. In the coming years,
the U.S. EPA and the Agency for Toxic Substances and Disease Registry are expected
to release further guidance on chemical mixtures risk assessments (34).
In this article, an update of our previous one (1), we review recent
studies and new developments concerning the toxicology and risk assessment of
chemical mixtures. Activities were grouped and will be discussed groupwise.
Conceptual Issues
CombiTool--a new computer program. CombiTool is a new computer
program for the analysis of the toxic effects of mixtures (35). It performs
model calculations and analyses of experimental combination effects for two
or three chemicals according to both Bliss independence (response addition)
and Loewe additivity (dose addition) criteria. These data can be displayed as
a difference response surface. They can be used as a general analysis of the
difference between Loewe additivity and Bliss independence. Zero interaction
response surfaces are calculated from single-chemical dose-response relationships
and compared with combination data obtained experimentally. CombiTool has a
graphics facility that allows direct visualization of the response surfaces
or the corresponding contour plots and the experimental data. As far as the
authors know (35), it is the only computer program that offers the possibility
of analyzing mixture studies according to Loewe additivity and Bliss independence
criteria. Earlier versions of CombiTool have already been used successfully
in several studies on combined-action assessment (36-39).
Mathematical basis for combination rules. A mathematical model
is being developed as a basis for combination rules that predict the effects
of chemical mixtures, starting from the concentrations of the individual chemicals
in the mixtures (40). This representational model begins with empirical
ordering of different mixtures of the same chemicals with respect to a relevant
adverse effect of the components in the mixture. This approach is taken from
measurement theory (41-43). Qualitative properties of the ordering
are postulated, then the combination rule that predicts this effect ordering
is mathematically derived. Different sets of properties imply different combination
rules. The valid set of properties for a certain mixture is found by empirically
testing whether the ordering satisfies the critical properties. An important
property is independence, which in the present approach is formulated in terms
of the effect ordering. This new approach is particularly useful if the insight
in the mechanism of toxicity of the individual chemicals is not sufficient for
predicting the effect of their mixture. It is meant as an alternative for the
use of combination rules based on classifications of the combined action with
classes such as independent similar action, independent dissimilar action, and
interaction. The representational approach is more rigorous and bases the selection
of a combination rule on testable postulates. A model consisting of four postulates
concerning the effect ordering of combined chemicals serves as an example (40).
Experimental Studies and Upcoming Technologies
Interaction of particles and gases in ambient air. Real-world
exposures to air pollutants are rarely to single pollutants but rather are a
mixture that reflects the integration of many sources, emission constituents,
or ongoing photochemical processes in the atmosphere. Apart from a range of
gases, the atmosphere also contains particulate matter (PM)--a mixture of solid
particles and liquid droplets that may vary in mass, size, and chemical composition,
depending on the sources and the meteorological conditions. Many components
may be adsorbed to the solid particles, for example, acids, partly neutralized
salts, aliphatic and (polycyclic) aromatic organic compounds, metals, and biomaterial
(allergens, pollen fragments, and endotoxins). PM levels relevant to human health
effects are commonly expressed on the basis of the mass concentration of inhalable
particles, defined to contain particles with an aerodynamic diameter ¾10
µm because only these particles can penetrate into the airways and lungs.
There is still debate about the validity of this type of metric; the number,
surface area, or specific chemicals might be a more relevant measure for setting
standards. Although air quality data have revealed that ambient aerosols have
typical trimodal mass or number distributions, showing peaks at coarse mode
(2.5-10 µm), fine mode (0.1-2.5 µm), and ultrafine mode
(<0.1 µm, dominated by the number of particles) particles (44),
PM is considered a complex mixture because of the large number of chemical constituents
in each of these classes.
Inhalation studies addressing the health effects of PM use either model compounds
predominantly present in the complex mixtures or the mixture itself, applying
the technology of concentrators (45). At present the prime goal of inhalation
studies is to confirm epidemiological associations between PM levels and observed
morbidity and mortality by showing a causal relationship. The identification
of the responsible component(s) is the secondary goal.
Synergistic interactions of pollutant gases and ambient PM have been studied
from the early 1950s. Amdur et al. (46) established that fossil fuel
irritants such as SO2 can interact physicochemically with soluble
metal salts to generate particles intrinsically more toxic than the primary
compounds. More recently, SO2 was shown to react with combustion-associated
ZnO emission PM in a humidified atmosphere, resulting in acid sulfate that can
be carried deep into the lungs of test animals (47,48). Other experimental
studies have supported the potential for combined gas-particle interactions
such as fine carbon or diesel, acidic, or dispersed ambient particles combined
with (in)organic gases or vapors (e.g., O3, NO2, SO2,
HNO3, aldehydes) (49-51). The results of these studies
suggest that particles can act as reactants or carriers to deliver toxicants
to the deep lung. A restricted number of human studies have been conducted similar
to Amdur's SO2 interaction experiments (52-54), using
acids and oxidants, but little evidence of synergism between PM and gases has
been observed, perhaps because of the small numbers of subjects tested in most
studies. Recently, in a 4-week inhalation study with 0.5-µm particles combined
with exposure to carbon black (50-100 µg/m3), ammonium
bisulfate (70 µg/m3), and ozone (0.15 ppm or 300 µg/m3),
the combination showed more deleterious effects than the components alone (55,56).
The effects included decreased alveolar macrophage function and increased lung
collagen concentration and lung cell turnover rates, although there were no
indications for increased lung permeability and inflammation. Acute inhalation
to ultrafine and fine carbon black in combination with ammonium nitrate in healthy
and compromised rats, however, did not reveal an interactive toxicological response
(57).
"Bottom-up" approach in chemosensory detection of mixtures. The
topic of chemosensory responses to mixtures has relevance not only to the basic
understanding of chemosenses but also to a variety of applied topics such as
food flavors and air quality (58,59). Many studies addressing the human
perception of chemical mixtures focus on olfaction and suprathreshold concentrations,
expressing results in terms of the response (59,60). Fewer studies focus
on threshold exposure levels and express the results in terms of the stimulus
(that is, concentration of the chemical) (59,61). Using binary mixtures
of 1-butanol and 2-heptanone, Cometto-Muñiz et al. (59) developed
a strategy for testing chemosensory detectability of mixtures by measuring detectability
functions for odor, nasal pungency, and eye irritation in normosmics and anosmics.
The results provide support for the existence of dose additivity in the detection
of chemical mixtures at perithreshold exposure levels. This appeared to hold
for all three end points (odor, nasal pungency, eye irritation). The data for
odor suggested that perithreshold stimulation might elicit little or no mutual
inhibition between components in the mixture. At levels progressively above
the threshold, inhibitory interaction (competitive agonism) appeared to grow.
At very low concentrations of an odorant mixture, there might be negligible
competition between components for binding to receptors (59). As the
concentration of the odorant in the mixture increases, the competition for binding
to the olfactory receptors also increases. The structural similarity between
odorants in a mixture, and thus their ability to bind to a smaller or larger
overlapping family of receptors, then becomes a crucial factor for understanding
the type of combined effects of the chemicals in the mixture (59). In
this respect it is of interest to refer to the observation of Cassee et al.
(62) that nasal sensory irritation in rats exposed to a mixture of formaldehyde,
acrolein, and acetaldehyde could be predicted by a model for competitive agonism,
thus providing evidence that the combined effect of these aldehydes is basically
a result of competition for a common trigeminal nerve receptor.
Only a systematic study of a number of binary mixtures in which the components
differ in structure from one another to various degrees can answer the question
of whether an increasing degree of molecular difference between components will
reduce the competition for the same receptor and thus also the degree of competitive
agonism in mixtures to produce odor detection. This issue will be addressed
in future studies (59).
The above approach of studying chemosensory detection of mixtures compared
with detection of the separate components via measuring complete detectability
functions (that is, measuring functions for both odor, nasal pungency, and eye
irritation) is a classic bottom-up approach, which indeed might take much time
and effort before providing data relevant to real-life mixtures consisting of
dozens of chemicals. Nevertheless, it is worth the cost and effort because this
approach may lead to a better understanding of the chemosensory impact of certain
mixtures such as volatile organic compounds (VOCs) (59).
Pattern recognition in safety evaluation of complex mixtures. In
case of complex mixtures that have not been properly tested toxicologically
and that may contain large numbers of unidentified components, pattern recognition
techniques may be used. Principal component analysis (PCA) is used to detail
the chemical characteristics of a mixture by comparing it with other mixtures,
either as fingerprints or as detailed information on identity and quantity of
each component (63). This kind of comparison of composition patterns
requires a database with compositional information obtained in a standardized
way. PCA has been used to analyze gene expression data from DNA arrays (64).
PCA appeared to be able to identify broad patterns of expression alterations.
Using PCA, genes could be clustered into related expression patterns (65).
A more sophisticated use of pattern recognition is to directly derive from
existing toxicity data on certain mixtures imaginary toxicity data for the complex
mixture of concern. This requires the use of multivariate regression techniques,
for example, projections to latent structures (PLS) (63). With PLS the
toxicity of a complex mixture can be predicted using the toxicity and the physicochemical
data of other complex mixtures along with the physicochemical data of the mixture
of concern. An advantage of this approach is that there is no need to explicitly
identify a complex chemical mixture that is chemically very similar to the mixture
of concern.
Overall, in the hands of experts, pattern recognition techniques are considered
powerful tools for the safety evaluation of complex chemical mixtures.
Toxicogenomics in mixture research. Advances in gene expression
technology provide the means to profile expression of thousands of messenger
RNAs simultaneously, and similarly, the expression of proteins within a cell
(66-68). Toxicology will benefit enormously from the application
of genomics (transcriptomics, proteomics, metabolomics) to analyze chemically
induced alterations in gene expression. Because of the integrated and holistic
nature of genomics, in all likelihood such changes in gene expression can be
identified at exposure levels lower than those affecting more conventional parameters
(34,69). Once validated, the use of combined transcriptomics,
proteomics, and metabolomics will make it possible to map early toxicity-related
alterations in cells, tissues, or animals exposed to chemicals, and thus will
lead to insight in numerous toxicologically relevant cellular processes simultaneously.
Clearly, validation studies are crucial, and indeed, some have been performed
(66,70,71). However, many more will have to follow to understand the
strengths and limitations of this new very promising technology (72).
The use of gene expression technologies such as microarrays (gene/DNA chips)
is most suitable to detect joint or interactive effects of chemical mixtures.
As part of a program on the validation of the weight of evidence (WOE) methodology
for assessing hazard and risk of defined mixtures, an oral subacute study in
rats that examines the toxicity of lead, mercury, benzene, and trichloroethylene
alone or as a mixture is under way (73). In addition to conventional
toxicity parameters, changes in messenger RNAs and protein expression levels
are being measured in various target tissues.
Strategic and Regulatory Issues
Drinking water disinfection byproducts. Drinking water treated
with chemical disinfectants such as chlorine and ozone contains disinfection
byproducts (DBPs) formed as a consequence of the reaction between the disinfectants
and natural organic matter present in the source water (74). Toxicological
and epidemiological studies indicate DBPs may cause adverse effects in humans
(10,75). To properly assess the toxicity and potential adverse health
effects of DBPs, researchers widely recognize that drinking water (treated with
disinfectants) should be approached as a variable, complex, very diluted chemical
mixture (75-77) with the following main characteristics: a)
large numbers of chemicals occurring at very low levels, b) a large fraction
(about 50%) of unidentified DBPs, and c) lifetime exposure of the consumer.
Different complex mixtures require different approaches to evaluate their safety,
and the usefulness of a certain approach depends on the context in which one
is confronted with the mixture and also on the amount, type, and quality of
the available data on the chemistry and toxicity of the mixture. Mixtures may
be virtually unavailable or readily available for testing in their entirety.
A possible approach is to focus on the most risky chemicals (10, for example)
in a mixture, assuming the risk of the entire mixture is largely determined
by the risk of the mixture of these selected components (77,78).
The safety evaluation of DBPs in drinking water is extremely complex. It has,
nevertheless, a very high priority because of the huge exposure in terms of
exposure time and number of individuals exposed. As a consequence, there is
a tendency to develop all-embracing research programs using every methodological
tool available (79,80). We feel this is an unrealistic and thus little
helpful strategy. We agree with Groten (77) that prioritization of the
various groups of DBPs is badly needed, focusing on an approach that considers
the large fraction of unidentified DBPs.
Public health effects of large airports. Recently, the Health
Council of the Netherlands published two reports addressing the topic of large
airports and public health (81,82). Quality of life was included in the
definition of public health. The key question was "Do large-airport operations
affect public health?" The answer was "yes." Major factors were air pollution,
noise, and safety, with aircraft noise generally being considered to have the
most significant impact on people living in the vicinity of a large airport
(81,83). Clearly, environmental factors other than chemical agents are
of major significance.
Air pollution around large airports is comparable to that in urban areas,
and thus the health consequences are also similar viz. increased mortality,
decreased life expectancy, increased cardiovascular and respiratory tract complaints,
and odor annoyance. Noise caused by large airport operations may lead to hypertension,
ischemic heart disease, decreased study performance, sleep disturbance, and
general annoyance (disgust, anger, dissatisfaction, resentment, discomfort)
(84,85). Fear and anxiety about aircraft crashes impact on quality of
life and contribute to stress. Other factors with possible impact on public
health are soil and water pollution, import of infectious diseases (malaria,
for instance), landscape appearance, and perception of risk and external safety.
The report ended with an attempt to integrate the findings. Is the impact
of cumulative exposures the sum of the effects of the separate exposures, or
is the cumulative exposure characterized by supra-additivity (or infra-additivity)?
This question could not be answered. Studies addressing the question were not
available in the scientific literature. Under the supervision of the National
Institute of Public Health and the Environment in the Netherlands, such a study
is currently being performed (86).
Safety evaluation of natural flavoring complexes. Natural flavoring
complexes are mixtures of constituents obtained by applying physical separation
methods to botanical sources. Sources include pulp, bark, peel, leaf, and flower
of fruits, vegetables, spices, and other plants. Many of the approximately 300
natural flavor complexes have a food origin, for example, lemon, basil, and
celery seed oils.
The method for the safety evaluation of natural flavoring complexes (the naturals
paradigm) is intended only for the safety evaluation of natural flavor complexes
derived from higher plants to be used as flavoring substances for food and beverages.
The naturals paradigm is a procedure that begins with a review of available
data on the history of dietary use of the natural complex, then prioritizes
constituents according to their relative intake (from use of the natural complex
as a flavoring substance) and their chemical structure (87). The method
further uses the concept of threshold of toxicological concern (88) and
assigns constituents to one of three structural classes (89-91).
Another aspect of the naturals paradigm involves the evaluation of constituents
of unknown chemical structure. As a conservative default assumption, the total
intake of all unknowns is considered together and placed in the structural class
of greatest toxic potential and thus compared with the most conservative exposure
threshold. The paradigm also addresses the concept of joint action among structurally
related constituents. If a common pathway of intoxication has been identified
or can be reasonably predicted on the basis of structure-activity relationships
for a group of constituents, the combined intake of those substances will be
compared with the appropriate human exposure threshold of concern. Ultimately,
the procedure focuses on those constituents or groups of constituents that,
because of their intake and structure, may pose significant risk from consumption
of the natural complex. With the developed strategy, the overall objective of
the naturals paradigm can be attained--that no reasonably significant risk associated
with the intake of natural complexes will go unevaluated. A publication describing
in detail the different steps of the naturals paradigm and containing example
evaluations is in progress (87).
Combined intake of food additives. Food additives are authorized
in the European Union (EU) on the basis that they constitute no health risk
to the consumer at the proposed level of use. Although additives at their permitted
use levels are considered safe, there are concerns that simultaneous intake
of different additives could be of potential health significance. Therefore,
the International Life Sciences Institute Europe Acceptable Daily Intake Task
Force established an expert group of independent scientists to analyze the possibility
of health implications of joint actions and interactions between the 350 food
additives currently approved in the EU (92). All approved additives allocated
a numerical acceptable daily intake value were studied. Target organs were identified
on the basis of the effects reported at doses above the no-observed-adverse-effect
levels in animal or human studies. Descriptions of the pathological and other
changes reported were used to assess whether different additives sharing the
same target organ would produce a common toxic effect. In all but a very few
cases, the possibility of joint actions or interactions could be excluded on
scientific grounds. The exceptions were some additives with effects on the liver
(curcumin, thiabendazole, propyl gallate, and butyl hydroxy toluene), the kidneys
(diphenyl, o-phenylphenol, and ferrocyanide salts), the blood (azorubine
and propyl gallate), and the thyroid (erythrosine, thiabendazole, nitrate).
In-depth consideration of both the specific use and the intake levels of these
last-mentioned additives led to the conclusion that joint actions or interactions
among these additives are a theoretical rather than a practical concern.
When approving future additives that show target organ toxicity, investigators
should consider the possible joint actions or interactions of previously approved
additives on the basis of a common mechanism of toxicity (92).
Nasal cancer associated with inhaled chemical mixtures. Nasal
cancer occurs in experimental animals after chronic exposure to a wide range
of inhaled chemicals (93,94). Although exposure to several of these chemicals
is common in industrial and domestic environments, epidemiological studies have
not provided convincing evidence that exposure to the individual chemicals is
associated with nasal cancer. The reverse seems to be true for inhalation of
chemical mixtures. The evidence for nasal carcinogenicity of inhaled mixtures
in experimental animals is very limited, whereas there is ample evidence that
occupational exposure to certain chemical mixtures is associated with increased
risk of nasal cancer (94). Examples of such carcinogenic (complex) chemical
mixtures are wood dust, textile dust, chromium-containing materials, and leather
dust. Whether wood-preserving agents contribute to the effects of wood dust
on the sinonasal mucosa has not yet been determined (95). Effects may
also be gender specific. A recent analysis revealed that nasal adenocarcinomas
due to wood-dust exposure are associated with a higher risk in men but not in
women, whereas exposure to leather dust is associated with an excess in both
genders (96). Moreover, tobacco smoking should not be overlooked as a
risk factor for sinonasal cancer, causing mainly squamous cell carcinomas (96,97).
On the other hand, a recent survey carried out in the United States has thrown
doubt on the significance of wood dust as a human carcinogen. Among men who
reported exposure to wood dust, there was an elevated risk of total mortality
but no excess of sinonasal cancer (98).
It is remarkable that these carcinogenic mixtures are aerosols, suggesting
that their particulate nature may be a factor in their potential to induce nasal
cancer in humans (94). Cigarette smoke as a complex mixture seems to
be an exception, as it was found to induce inflammation, degeneration of olfactory
epithelium, and hyper- and metaplasia of the nasal respiratory epithelium in
experimental animals (99-103). However, in all likelihood these
nasal effects are caused by vapor phase components such as formaldehyde, acetaldehyde,
acrolein, and furfural, and not by the particulate phase of cigarette smoke.
Whatever the identity of the responsible cigarette smoke components, these
findings in experimental animals correlate with the excess risk of sinonasal
squamous cell carcinoma observed in smokers in Europe (96). In this respect
it is also relevant to emphasize recent findings by Klein et al. (104)
that reveal 100% incidence of nasal tumors in rats after long-term exposure
by inhalation to 2.4 ppm 1-nitroso-4-methylpiperazine, resulting in a total
dose of 86 mg/rat. This was almost two orders of magnitude lower than the dose
inducing nasal tumors in rats after oral administration of this nitroso compound.
These findings suggest a major role for carcinogenic nitrosamines in tobacco
smoke upper and lower respiratory tract carcinogenesis.
Volatile organic compounds from building materials. To evaluate
and regulate emissions of VOCs from building materials, Nielsen et al. (105)
suggested the use of indoor air standards or guidelines, or when these are not
available, occupational exposure limits (OELs) divided by a default safety factor
of 40 or another factor when justifiable. A committee of the Health Council
of the Netherlands (106) considered the predictive value of OELs for
assessing the potential health effects of emissions from building materials
too low to justify their use for this purpose. According to this committee,
the exposure period (8 hr/day; 5 days/week; 40 years) and the target population
(workers) differ too much from the indoor environment situation. This committee
also discussed the use of air quality guidelines developed by the World Health
Organization for outdoor air (107) but advised against their use for
a practical reason: such guidelines have been established for only a few VOCs.
The committee recommended the use of the chemosensory effect of VOCs as the
critical effect and as a basis for the derivation of a recommended limit value
for VOCs in indoor air. The committee estimated the maximum tolerable pollution
of indoor air by VOCs to be between 0.2 and 3.0 mg/m3 and recommended
a limit value of 0.2 mg/m3 for VOCs as a mixture (106). The
committee emphasized that this value does not take into account potential health
risks attributable to individual VOCs with known carcinogenic, reprotoxic, or
sensitizing properties. VOCs possessing such properties should not be used in
building materials, and when their use is unavoidable, a separate risk assessment
should be performed.
Harmonized hazard classification criteria for mixtures. In November
1994, the 22nd Joint Meeting of the Chemicals Committee and the Working Party
on Chemicals, Pesticides and Biotechnology of the Organisation for Economic
Co-operation and Development (OECD) created the Programme on Harmonization of
Classification and Labelling (HCL). The objective of this program was to develop
a harmonized classification system for chemical substances and mixtures. The
work on this classification system for mixtures comprised the following eight
hazard end points: acute toxicity, skin and eye corrosion/irritation, respiratory
or skin sensitization, germ cell mutagenicity, carcinogenicity, reproductive
toxicity, specific target organ systemic toxicity, and hazards for the aquatic
environment. A detailed review document (DRD), "Hazard Classification for Chemical
Mixtures in OECD Countries," was developed and formally approved by the 9th
Meeting of the Task Force on HCL in February 2000 (108). This DRD is
available as document ENV/JM/HCL(99)10/REV2 Part 1 and Part 2. The DRD on mixtures
was further discussed, and drafting groups prepared chapters on each of the
eight hazard end points and a chapter titled "General Introduction and Considerations"
(108). In September 2000, the 10th Meeting of the Task Force on HCL reached
full consensus on the chapter "General Introduction and Considerations" and
on all but two chapters on the various hazard end points. The two outstanding
chapters were on skin and eye corrosion/irritation and hazards for the aquatic
environment. Following two joint meetings of the Chemicals Committee and the
Working Party on Chemicals, Pesticides and Biotechnology, one held in November
2000 and the other in June 2001, the final version of the "Harmonized Hazard
Classification Criteria for Mixtures" was published as part of the document
"Harmonized Integrated Classification System for Human Health and Environmental
Hazards of Chemical Substances and Mixtures" in June 2001 (109).
Discussion
The present survey deals with a variety of mixture studies ranging from the
development of a new computer program (35) and a new mathematical model
as the foundation for rules predicting the toxicity of mixtures (40)
to the application of gene expression technologies for detecting joint or interactive
effects (34,69). Moreover, risk assessment of real-life mixtures such
as the simultaneous intake of food additives (92), combined exposure
to fine particles and gases in ambient air (46-57), and DBPs in
drinking water (77,79,80) were addressed. Attention was also drawn to
strategies for estimating public health effects of large airports (81,82,86)
and the development of harmonized hazard classification criteria for chemical
mixtures (108,109).
The topics discussed varied greatly, which is not surprising because humans
are exposed concurrently and sequentially to hundreds of thousands of chemicals
from very different sources such as food, drinking water, beverages, indoor
and outdoor air, soil, and consumer products. Thus, mixed exposures are everywhere
and are the rule rather than the exception, indicating exposure assessment,
hazard identification, risk assessment, and risk characterization should focus
on mixtures rather than on single chemicals. However, until recently, about
95% of the sources in toxicology were devoted to the exception, namely, single
compounds (110,111). Although all of this is true, there is an alternative
way to look at exposure to chemical mixtures. Humans (and animals) apparently
have learned to cope with simultaneous exposure to huge numbers of chemicals.
In fact, exposure to certain mixtures, for example, mixtures of essential nutrients,
drinking water, and air, are vital. Moreover, one might wonder whether ideally
our food should be exclusively composed of a mixture of pure essential nutrients,
and our drinking water of pure H2O, and the air we breathe of pure
oxygen or a mixture of pure oxygen and nitrogen. Such pure environments are
unrealities and therefore should not be pursued. This implies that the focus
in toxicology should not be on mixtures (and chemicals) but on priority mixtures
(and priority chemicals), with priority being determined by (potential) risk
(= toxicity and exposure), i.e., the smaller the (presumed) margin of safety
(the ratio exposure level to health-based limit value might even be >1),
the higher the priority. To set such priorities, choices have to be made on
the basis of data or educated guesses. Because resources for (mixture) research
are limited, risks based on perception only should not be considered and realism
should outweigh purism.
Finally, we draw attention to the work of an ad hoc Committee of the Health
Council of the Netherlands that has just finalized a report on assessment of
health effects of exposure to combinations of substances (112). The report
presents a framework for health risk assessment of exposure to combinations
of chemicals. Two conspicuous elements of this framework are the distinction
between mixtures and specified combinations of substances, and the use of the
"top n" and "pseudo top n" approaches. For prioritization of mixtures
or combinations of chemicals, the report also includes the Mumtaz and Durkin
WOE approach (113,114).
This overview highlights some international issues on the toxicology of mixtures.
Clearly, strategies to tackle the safety evaluation of combined exposures and
complex chemical mixtures, as well as models facilitating the interpretation
of findings in the context of risk assessment of mixtures, have become increasingly
important.
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