Absorption rate pesticides

Another technique is to monitor drug or toxicant excretion rather than blood concentrations, especially when blood or plasma concentrations are very low. Using the same equations, the AUC is now replaced by chemical concentrations in urine, feces, and expired air. Some chemicals are primarily excreted by the kidney and urine data alone may be necessary. The rate and extent of absorption are clearly important for therapeutic and toxicological considerations. For example, different formulations of the same pesticide can change the absorption rate in skin or gastrointestinal tract, and not bioavailability, but can result in blood concentrations near the toxic dose. Also different formulations can result in similar absorption rates but different bioavailability. 

In a majority of instances, particularly with pesticides, combinations of chemicals (e.g., organic solvents, mineral oils) are mixed to enhance the chemical toxicity. Consequently, the toxicity profile is altered. Lipophilicity modulates the absorption rate of the chemical from the site of entry into the system (lung, skin, mucous membrane). Thus, the fat-solubility pattern of a test chemical helps for easy cell membrane transport to reach the active site of intracellular enzymes and trigger possible toxic effects. The toxicity profile of active ingredients of pesticides and those of formulated products of pesticides differ widely. 

It is commonly known that the skin contains a large range of enzymes capable of metabolizing topically applied compounds. Eor pesticides, esterase activity is among the most important (van de Sandt et al, 1993 Hewitt et al, 2000). Although the stratum comeum is generally accepted as the most important barrier in skin absorption, there are some indications that skin metabolism in other skin layers influences the percutaneous absorption of compounds (Potts etal, 1989). The interrelation between metabolism and absorption rate, however, has not been unequivocally established. 

Despite a 3-fold range in follicle area in the marmoset, no differences in absorption rates of paraquat, mannitol, water, and ethanol were observed between different body sites (Scoll et al., 1991). However, among the different species examined in this study, there was an 80-fold range in follicle area, which correlated with observed differences in the rate of mannitol and paraquat absorption. The authors concluded that this correlation was only possible with relatively slowly ab.sorbed le.st penetrants, such as paraquat and mannitol. Further work is needed to determine to what extent unique anatomical features at different body sites play a role in absorption and penetration of both lipophilic and hydrophilic pesticides. 

The presence of fonnulation surfactants and solvents, which will facilitate skin penetration of the pesticide. Absorption rate is more effective for lipophilic materials. Some U V-absorbing chemicals can act as skin penetration enhancers, which may increase the pcrcutancou.s aKsoiption of pesticides and other formulation chemicals (Morgan... 

Anatomical. site contamination. With many materials, including pesticide.s, the absorption rate through the skin of a specific material varies with the regional location of the skin area contaminated. For example, with parathion the absorption rate is faster through skin of the scrotum, axillae, and face than through skin of the hands and anns (Maibach et al., 1971). Also, Maibach and Feldman (1974) applied 4 p,g cm of parathion to the forearm, abdomen, and forehead of volunteers and found that the absorbed doses were 8.6, 18.5, and 36.3%, respectively. 

New Research. Regarding new research, the Department of Labor and the Environmental Protection Agency plan to co-sponsor a five-year study of the effects of pesticide exposure on the health of youths under sixteen years of age who are employed in agricultural operations. The study will be undertaken to determine a) actual pesticide exposure and physical effects of such exposure b) absorption rates of pesticides into the body, and c) acute and chronic health effects in relation to duration and level of exposure. 

In order to assess the risk from topical exposure a number of investigators have sought animal models that could predict percutaneous absorption rates of chemicals in humans. Considerable efforts by Wester and Maibach (2-6) have shown that monkeys and pigs give dermal absorption data most comparable to humans with a range of drugs and pesticides which varied in their physicochemical properties as well as use. A similar rank order for species comparisons has been observed in in vitro (12-14) absorption data which in most cases exceeded the human values (Table I). For this reason, and because of the availability of rhesus monkeys within our facility, dermal absorption studies with rhesus monkeys were considered an appropriate model. 

Laboratory Absorption Studies Several techniques are available for estimating dermal absorption of pesticides. These techniques are extensively reviewed in an earlier section of this book. For most pesticides, a reasonable estimate of dermal absorption for humans can be made using laboratory data. These data are used to estimate an average dermal absorption rate from tihich one can calculate the percentage of active ingredient that is absorbed in a typical eight-hour work day. 

The transport processes that may move disulfoton from soil to other media are volatilization, leaching, runoff, and absorption by plants. Volatilization of disulfoton from wet soil may be greater than from relatively dry soil (Gohre and Miller 1986). Like other pesticides, disulfoton in soil partitions between soil-sorbed and soil-water phases (Racke 1992). This latter phase may be responsible for the volatilization of disulfoton from soil however, due to the low Henry s law constant value, the rate of disulfoton volatilization from the soil-water phase to the atmosphere would be low. 

The apparent rate of excretion was slower after dermal exposure than after oral administration, probably due to slower absorption of the 2,4,5-T ester from the skin than 2,4,5-T acid from the gut. This is in agreement with observations made by Feldmann and Maibach for 2,4-D and other pesticides applied to the forearm of human volunteers (13). Calculations by Ramsey et al. using three methods showed that 97% of the 2,4,5-T absorbed by forest workers would be excreted in urine within 7 days following dermal exposure under typical field conditions (16). 

Dermal Exposure Of the three major routes of exposure, the dermal (skin) route constitutes nearly 90% of chemical exposure, particularly of pesticides. Dermal exposure is common whenever chemicals are mixed or handled. Certain types of dry materials, (e.g., pesticide dusts, wet or dry powders, granules, liquid pesticides) enter the body through quick skin absorption. Many factors influence the rate of dermal exposure of a chemical these may be as follows ... 

A method to set REIs would account for the rate of dermal absorption, the rate of foliar contact and the rate of change in cholinesterase. These factors were used in the Popendorf and Leffingwell (1982) Unified Field Model for determining REIs. This model also accounts for the relative rate of DFR dissipation, and differences in potency based on the dermal LD50 of the pesticide. The Unified Field Model is an elegant technique that takes into account many variables affecting exposure and cholinesterase inhibition as a response. Ultimately, the rate of cholinesterase inhibition, and not a fixed level of inhibition, is the primary... 

The UK-POEM database is based on a review of the data available on the exposure of pesticide spray operators (in the UK). The review indicated that several factors determined the dose absorbed by a spray operator. These included the following the volume of external contamination, the extent to which this external contamination penetrated clothing to reach the skin and the rate at which the chemical came into direct contact with the skin surface and was absorbed (JMP, 1986 Martin, 1990). These various independent factors were assumed, with the exception of dermal absorption, to be of a sufficient generic nature to be suitable for extrapolation purposes. Two major work activities were differentiated mix-ing/loading and application. An update of the default values in UK-POEM has been presented (POEM, 1992). 

Fig. 4.29. Extraction-time profile curves (signal height versus absorption time) for organochlorine pesticides in aqueous solutions. Conditions spiking level 0.2-1.2 ng/ml polydimethylsiloxane fibre length 100 pm magnetic stirring rate 250 rpm. Although equilibrium was not reached for some analytes, the extraction time was chosen to be 15 min. ( ) a-hexachlorocyclohexane, ( ) /3-hexa-chlorocyclohexane, (A) dieldrin, (x) aldrin, (O) 4,4 -DDT, ( ) 4,4 -DDE, ( + ) 4,4 -DDD and ( ) endosulfan sulphate. (Reproduced with permission of Elsevier.)...

The need to match the UV absorption spectrum of the pesticide, plus developing a solution that fits the use profile of the product (e.g. soil-applied, vs. foliar-applied, etc.) means that most solutions to UV stability are particular to the pesticide protected. A significant obstacle to the extensive use of UV stabilisers is their often significant cost (relative to the pesticide), as well as the high application rates applied. Furthermore, the improvements seen to date have... 

Baker s yeast cells Saccharomices cerevisae) were successfully immobilized on to silica gel and used in selective online trace enrichment of selected pesticides, including linuron, in various types of natural waters.This technique relies upon the fact that microorganisms are able to absorb pesticides from water in the environment. Cell membranes contain many classes of lipids and lipoproteins, which contribute to pesticide absorption. Since the diffusion rate across membranes is inversely proportional to molecular size, low-molecular weight compounds, such as pesticides, can be extracted from water and isolated by naturally occurring high-molecular weight substances, such as humic acids, which are abundantly present in environmental waters. 

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