Combined Actions and Interactions of Chemicals in Mixtures

The cytotoxicity may be general, non-specific, and the effects will depend on the
distribution of the toxic substance in the respiratory tract. The water solubility of the
compound is one of the major determinants of the site of action. If the toxic agent is
an aerosol, the prime determinant of the site of action is the particle size.
7.1.4.2 Test systems
Respiratory tract toxicity is most often modelled in animal experiments or evaluated
on the basis of experiments with human volunteers. Only few studies exists on
combined toxic effects using in vitro systems. Human alveolar macrophages have,
for example, been simultaneously exposed to NO2 and particles or fibres. The
cytotoxic effects of the combined exposure were higher than particle or fibre
induced cytotoxicity. Co-exposure did, however, reduce the cellular expression of
proinflammatory cytokines (Drumm et al., 1999).
Respiratory tract toxicity is extensively being predicted by physicochemical
modelling (Ultman, 1988; Gargas et al., 1993). This approach may be combined with
in vitro studies of respiratory tract irritancy using cell cultures, isolated human or
animal tissues or reconstructed tissues.
With respect to harmful effects in connection with aspiration risk to the lung after
ingestion, oil-/coal- derived substances (or preparations) are classified according to
physical-chemical properties with regard to its flow rate and viscosity.
7.1.4.3 Examples of combined action
Ozone and nitrogen dioxide are examples of toxic agents that may produce cellular
damage and synergistic effects in the respiratory tract. The two gases are among the
most critical irritants in urban air. The water solubility of both ozone and nitrogen
dioxide is sufficiently low that the main site of action is the respiratory bronchioles
and alveoli, and exposure to either of the gases is well known to produce a variety of
morphological and biochemical changes in the lung.
Using animal models it has been demonstrated that combined exposure to ozone and
nitrogen dioxide resulted in morphological changes in the airways, which were
principally accounted for by ozone alone (Freeman et al., 1974), biochemical effects
that were additive (Yokoyama et al., 1980; Mustafa et al., 1984), or acute effects
(pulmonary oedema and mortality) which were synergistic (Diggle & Gage, 1955).
However, recent experiments with combined exposure of rats to varying levels of
ozone (400-1600 mg/m 3 ) and nitrogen dioxide (6840-27360 mg/m 3 ) indicate that a
threshold level for synergistic effects may exist. Clear synergistic effects were
observed on the recovery of cells and proteins obtained by pulmonary lavage of
combined exposure to ozone and nitrogen oxide levels of 800 mg/m 3 and 13680
mg/m 3 , respectively, and at higher exposure levels. However, the effect was less than
additive at the lowest level of combined exposure (Gelzleichter et al., 1992).
The most likely mode of action of ozone and nitrogen dioxide is through peroxidation
of cellular membranes. By combined exposure of rats to low levels of ozone (100
mg/m 3 ) and nitrogen dioxide (76 mg/m 3 ) during 5 months, synergistic effects
regarding lipid peroxidation of the lung tissue were observed (Sagai & Ichinose,
1991). Based on results from an in vitro experiment with combined exposure of
human red blood cells to ozone and nitrogen dioxide, it was concluded that the
absolute and relative concentrations of the two gases as well as the time course and
the sequence of administration modified the occurrence of toxic effects. Co-exposure
to ozone and nitrogen oxide induced additive effects on the osmotic fragility of the
cells, lipid peroxidation, acetyl cholinesterase activity and levels of reduced
glutathione and methemoglobin. At low levels of the gases, a synergistic increase in
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