Michael S. McLachlan

Atmospheric deposition of semivolatile organic compounds to two forest canopies

Abstract The deposition of polycyclic aromatic hydrocarbons, hexachlorobenzene and polychlorinated biphenyls, dibenzo-p-dioxins and dibenzofurans to a coniferous and a deciduous forest canopy was measured by simultaneously sampling bulk deposition below the canopies and in an adjacent clearing for one year. In addition, ambient air was sampled continuously, with separate analysis of the gaseous and particle-bound phases. The deposition of almost all compounds was higher under the forest canopies than in the clearing. The excess deposition to the forest sites was attributable to equilibrium partitioning between the atmosphere and the canopy vegetation, kinetically limited gaseous deposition, or particle-bound deposition. Which of these deposition processes dominated for a given compound was shown to depend on the octanol–air partition coefficient of the chemical and its gas/particle partitioning. Deposition velocities–to our knowledge the first for SOCs to forests–were calculated by dividing the excess deposition by the air concentrations. The gaseous deposition velocities were 0.78xa0cmxa0s-1 to the coniferous canopy (annual weighted average) and 3.6xa0cmxa0s-1 to the deciduous canopy (6 month summer average). These values are high compared to deposition velocities to forest canopies that have been measured for inorganic gases, reflecting the fact that lipophilic organic chemicals are taken up by the leaf/needle cuticle and not just via the stomata. The dry particle bound deposition velocities for particle diffusion and impaction were 0.05 and 0.73xa0cmxa0s-1 for the coniferous and deciduous canopies, respectively. These values are considerably lower than the gaseous deposition velocities, underlining the importance of gaseous deposition for the accumulation of semivolatile organic compounds in forest ecosystems.

Gas/particle partitioning of PCDD/Fs, PCBs, PCNs and PAHs

In a study to establish the baseline contamination of the environment prior to the start-up of a new municipal incinerator, air samples were collected continuously over a period of one year at eight locations in southern Bavaria. The air samples were analysed in six week intervals for the particle bound and gaseous concentrations of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs) and naphthalenes (PCNs) as well as polycyclic aromatic hydrocarbons (PAHs). n nThe logarithms of the particle/gas partition coefficients (Kp) were closely correlated with the inverse mean ambient temperature (r2 > 0.9 in most cases). When log Kp at 25°C was plotted vs. the logarithm of the compounds subcooled liquid vapour pressure (PL), the investigated compounds separated out into two groups. Within each of the two groups a linear correlation between Kp and PL was observed, but for the PAHs and PCNs the particle bound fractions were always considerably higher than for the PCDD/Fs and PCBs with comparable PL. These differences indicate that PL is not a good parameter for describing the interchemical variability in gas/particle partitioning. n nLog Kp was also plotted against the logarithm of the octanol/air partition coefficient (KOA), and in this case the different chemical groups did not separate out on the plot. This suggests that KOA may be a better descriptor of partitioning behaviour for these substances. However, the r2 for the log Kp vs. log KOA plot was identical to that for the log Kp vs. log PL plot (0.77), and hence no conclusions could be drawn about the relative utility of the two properties. Future improvements in the KOA data base may help to resolve this issue.

Atmospheric particle size distributions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) and their implications for wet and dry deposition

Abstract Concurrent measurements of polychlorinated dibenzo- p -dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) in different size fractions of atmospheric particulate matter are presented for a winter and a summer sampling period. The PCDD/Fs and PAHs were primarily associated with particles of μ m aerodynamic diameter. The particle size distributions were similar for the compounds within each substance group and, surprisingly, also between the PCDD/Fs and PAHs. Changes in the particle size distribution of particle mass were reflected in the particle size distributions of the PCDD/Fs and PAHs. The data were employed to identify those particle size fractions dominating the wet and dry particle bound deposition of PCDD/Fs and PAHs and, furthermore, to assess the relative contributions of wet and dry deposition to the total particle bound deposition fluxes. The calculations indicate that coarse particles contribute most to the dry deposition while, in contrast, the wet deposition of the PCDD/Fs and PAHs is dominated by fine particles. Furthermore, it is estimated that in Bayreuth wet deposition dominates the total particle bound deposition of PCDD/Fs and PAHs.

The influence of dietary concentration on the absorption and excretion of persistent lipophilic organic pollutants in the human intestinal tract.

The gastrointestinal exchange of polychlorinated biphenyls (PCBs), dibenzo-p-dioxins, and dibenzofurans (PCDD/Fs) as well as hexachlorobenzene was measured in five volunteers. The dietary intake and the fecal excretion of the chemicals were quantified and the net absorption/net excretion was calculated as the difference between these two fluxes. Experiments were conducted using an elevated dietary intake and a reduced dietary intake of chemical, and the results were compared with the absorption during normal dietary intake. The net absorption varied widely with the dietary intake for those compounds which bioaccumulate in humans; high dietary intake of chemical resulted in absorption approaching 100% of intake, while low dietary intake resulted in a net excretion several times greater than the dietary intake. In contrast to net absorption, the chemical flux in the feces was largely independent of the dietary intake of chemical for a given individual. Good agreement was found between the feces/blood distribution coefficients measured in this study and in a study with contaminated workers whose blood concentrations were several orders of magnitude higher, indicating that fecal excretion of chemical is linearly proportional to the blood concentration. The results suggest that gastrointestinal exchange can be viewed as two processes operating simultaneously: absorption of contaminant from the diet, and excretion of contaminant from the bodys reservoirs via the feces. By subtracting that component of the fecal flux originating from the body, the maximum dietary absorption could be calculated. This was >95% for most of the compounds, decreasing to a minimum of 50-60% for the octachlorinated dioxins and furans. The maximum dietary absorption showed a Kow dependency consistent with the two film model of gastrointestinal absorption of persistent organic chemicals.

Distribution of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) within the full size range of atmospheric particles

Abstract Particle size distributions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs) were measured at a rural location in northeastern Bavaria during summer. The complete size range of the atmospheric particles was sampled using cascade impactors and a rotary impactor. On an average 92% of the hepta- and octachlorinated PCDD/Fs and 97% of the PAHs with five or more rings, which were all more or less completely particle bound in the atmosphere, were associated with particles with aerodynamic diameters (dae) 8.6xa0μm. The temporal variation of the size distributions was low. Despite similar measured size distributions the washout ratios of the PCDD/Fs and PAHs calculated using simultaneously collected precipitation samples were different, with those of the PAHs lying consistently lower than those of the PCDD/Fs. This may have been due to differences in the particle size distributions which were not resolved with the impactors used. In the case of the semivolatile compounds for which significant fractions were present in the gas phase, 60–95% of the particle bound substance mass was always found on particles with dae

PCDD/F in an agricultural food chain. Part 1: PCDD/F mass balance of a lactating cow.

Abstract A mass balance was conducted of the PCDD/F fluxes into and out of a lactating cow in an unmodified environment. Feed was found to be responsible for virtually all of the cows PCDD/F exposure. Approximately 20% of the 2,3,7,8-Cl4DD toxic equivalents taken up by the cow was excreted in the milk. The measured transfer rates for a cow in a natural state generally agreed well with the results from feeding experiments in the literature. The digestive tract resorption of PCDD/F decreased with increasing degree of chlorination. Both the feces and the milk were important routes for the excretion of persistent PCDD/F isomers.

A non-absorbable dietary fat substitute enhances elimination of persistent lipophilic contaminants in humans.

For individuals contaminated with persistent lipophilic pollutants, there is an urgent need for a therapy to enhance contaminant elimination from the body and hence reduce long term exposure. This study investigated the possibility of enhancing the excretion of native chemical via the faeces by augmenting the lipophilic properties of the faeces with the non-absorbable lipid substitute olestra. The faecal excretion of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), and hexachlorobenzene (HCB) was measured in 3 volunteers. The excretion while eating an olestra-free diet was compared with the excretion while eating a diet supplemented wit 25 g/d of olestra. The excretion while on the olestra diet was higher by a factor of 1.5-11, depending on the compound. This resulted from higher concentrations of the contaminants in the faeces and higher excretion of faeces dry mass due to the food additive. Using 2,3,7,8-Cl4DD as an example, it was estimated that ingestion of 25 g/d of olestra would more than double the overall rate of elimination of this compound from the body. It is concluded that regular consumption of olestra may provide a therapeutic approach for reducing the body burden of persistent lipophilic contaminants.

Baseline contamination assessment for a new resource recovery facility in Germany part II: atmospheric concentrations of PCDD/F

The atmospheric levels of PCDD/Fs in both the gas and particle phases were measured continuously over 1 year at 7 sites in the outskirts of Augsburg, Germany, and at a rural site 15 km from the city. The homologue and congener patterns in the total air samples were similar among the different stations and stayed constant throughout the whole year. The PCDD/F levels were about two times higher at the sampling points on the edge of the city than at the remote location. There was a pronounced temporal variability of the total air concentrations, with levels in winter that were 9 times higher than the levels in summer. The gas/particle partitioning was characterised by higher gaseous fractions for more volatile compounds and higher ambient temperatures. It was concluded that the ambient air concentrations of the PCDD/Fs are determined by on-going local and regional emissions, with seasonal (winter) sources contributing the vast majority of the emissions fluxes.

Olestra increases faecal excretion of 2,3,7,8-tetrachlorodibenzo-p-dioxin

Two patients with chloracne had concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) of 144,000 and 26,000 pg/g blood lipids. Olestra, a non-digestible, lipophilic dietary fat substitute accelerated the patients intestinal excretion of TCDD by eight to ten fold. This is sufficient to reduce the normally observed elimination half life of TCDD from about 7 years to 1-2 years.

Clearance of PCDD/Fs via the gastrointestinal tract in occupationally exposed persons

A digestive tract mass balance was performed on six men with high body burdens of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Intake via food was measured by analyzing duplicate portions of the food consumed by the volunteers and excretion via feces was determined by quantitative collection and analysis of the feces. Blood samples were taken to determine the current body burden. The results showed that the quantity of non-metabolized chemical excreted in the feces clearly exceeded the uptake via food for all of the 2,3,7,8-substituted PCDDs and some of the PCDFs, indicating a significant clearance across the gastrointestinal tract. The concentrations of these PCDD/F congeners in blood and feces were highly correlated (r > 0.8), demonstrating that the fecal PCDD/F content was determined by the body burden. The half lives in the test persons due to fecal clearance of non-metabolized chemical were estimated from the excretion rate and the current body burden and ranged between 10 years (Cl8DD) and 33 years (2,3,4,7,8-Cl5DF). These were compared with the overall contaminant half-lives due to all clearance processes which were calculated from the body burden and the decrease in blood concentrations measured over several years. The fecal clearance of non-metabolized PCDD/F contributed on average between 37% (2,3,7,8-Cl4DD) and 90% (Cl8DD) to the total elimination. This indicates that the gastrointestinal pathway plays a decisive role in the clearance of most 2,3,7,8-subsituted PCDD/F congeners.