Palladium [Pd, 46]
239
Plants exposed to automobile exhausts contain Pd within from 0.4 to 2.4 μg/kg,
with the highest value for moss and the lowest for plantain (Dijingova et  al. vide
Kabata-Pendias 2011). The average Pd content in grass from roadsides in Poland is
3.2 μg/kg. Palladium contents in grass from roadside areas decrease with the distance from road 129 to 11 μg/kg, at 0 and 5 km, respectively. However, no relationship with its amounts in soils was observed (Hooda et al. vide Kabata-Pendias 2011).
The mean Pd content in cereal grains is calculated as 0.9 μg/kg FW, whereas in
food plants it varies from 0.4 to 3.0 μg/kg, being lowest in apples and highest in nuts
(WHO 2002a).
Stress effects of Pd in plants have been observed at a lower concentration, compared with other posttranslational modifications, apparently due to its relatively high
biochemical activity. It is likely to be bound to high-molecular-weight proteins and
is able to replace Mn in some metalloenzymes, due to similar ionic radii. Phytotoxic
effects in terrestrial plants are observed at Pd concentrations from about 2 to 60 mg/L
of the nutrient solution (WHO 2002a), but there are also reported toxic effect at its
lower concentration, from 1 to 3 mg/L (Kabata-Pendias 2011).
32.6 HUMANS
Concentrations of Pd in human wet tissue samples (liver, kidney, spleen, lung, muscle, fat) were below the limit after automotive catalytic converters (ACC) were not
used, since 1974. It was less than 0.6–6.7  μg/kg (Johnson et  al. 1976 vide WHO
2002a). Direct exposure to traffic has no verifiable influence on the background Pd
burden of the population. It is probable because the concentrations of Pd in the body
fluids of unexposed people are <0.1 μg/L in blood and <0.3 μg/L in urine (WHO
2002a).
Exposure of the general population is through Pd in air, food, and water, and
through release of Pd from dental restorations. Exposures to Pd and its compounds
(in dusts or solutions) may also occur in workers of the Pd mining, smelting, refining,
or recycling industries; the chemical industry, particularly in catalyst manufacture,
the electronics industry, and jewelry production.
Palladium ions can be taken up by the skin and by oral and inhalative routes.
Although its absorption and retention are poor, there may be a risk for sensitive
persons. Under certain conditions, Pd ions (and possibly microparticles) appear to
be released from metallic Pd (e.g., in dental alloys). There are also indications that
very finely dispersed element Pd particles become bioavailable, when dissolved in
biological media. However, precise quantitative data are not available.
Absorbed Pd is in almost all organs, tissue fluids of a body, with maximum in the
kidney, liver, spleen, lymph nodes, adrenal gland, lung, and bones (of experimental
animals). Palladium and its compounds are of very low to moderate acute toxicity if
swallowed (depending mainly on their solubility). Several Pd salts may cause severe
primary skin and eye irritations. The biological half-life of Pd in rats has been estimated to be 12 days (WHO 2002a).
A major source of health concern is the sensitization risk of Pd as very low doses
are sufficient to cause allergic reactions in susceptible individuals. Persons with
known Ni allergy may be especially susceptible. Workers occupationally exposed to