Analysing Work Environment and Chemical Risk Factors for Occupational Exposure to Pesticides in Horticulture
Date
2021
Authors
Tefera, Yonatal Mesfin
Editors
Advisors
Gaskin, Sharyn
Thredgold, Leigh
Thredgold, Leigh
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Thesis
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Abstract
Problem statement
Exposure to pesticides in agriculture constitutes a major occupational and public health
concern. Worldwide, it has been estimated that over a billion people work in agriculture, and
most use pesticides to protect their crops from insects and other pests. Every year, occupational
exposure to pesticides has been associated with more than 385 million poisoning cases, over
200,000 deaths, and a range of adverse health outcomes.
Greenhouses are enclosed structures with unique work environment characteristics, and the
implication of these factors for occupational exposure to pesticides is a major area of interest.
The use of personal protective equipment (PPE) such as gloves is among the key interventions
to minimise pesticide exposure in greenhouses. The selection of suitable gloves remains a
challenge and the chemical protection performance of gloves may be dependent on work
environment factors such as temperature, which is of increasing concern in a warming climate.
Organophosphate (OP) insecticides are among the most widely used classes of pesticides and
the skin is the primary route of occupational exposure in horticulture. Thus, there is a need to
understand dermal exposure and the glove permeation potential of this class of pesticides.
Dermal exposure assessments used for pesticide regulatory decisions have focused mostly on
the active ingredients (AI), largely ignoring the fact that workers in real-life encounter
commercially available formulated products and the potential role of formulation coingredients. Gap analysis
The following four research gaps were identified through reviewing the literature evidence
addressed in this research.
1. Greenhouses have some unique characteristics that set them apart from traditional open
field farms. However, little is known about the implications of these greenhouse
characteristics and the interactions therein on occupational pesticide exposure.
2. Although different chemical-resistant gloves are recommended for handling pesticides
in horticulture, the comparative performance of these gloves in warm to hot conditions
is understudied.
3. Apart from work environment factors, physicochemical and formulation attributes of
the pesticide product itself may influence dermal exposure as well as permeation
through chemical resistant gloves. However, the nature and extent of dermal exposure
and the relevance of physicochemical properties and skin permeability models in
predicting dermal absorption of pesticides from commercial formulations remain
unclear.
4. Although pesticides are often complex mixtures of one or more AI and several coingredients,
most in vitro dermal exposure assessment studies have only focused on the
AI. Very little is known about the role of formulation co-ingredients on dermal
absorption and glove permeation potential of the active insecticides.
Purpose statement
The overarching purpose of this research is to understand how work environment and chemical
risk factors influence occupational exposure to pesticides in horticulture. There are two primary
aims of this research: 1) A critical literature review attempts to examine how greenhouse
characteristics influence occupational exposure to pesticides, and 2) A series of laboratory-based experimental studies set out to understand the nature and extent of dermal exposure and
glove protection against OP insecticides as commercial formulations.
General research questions
1) What is the evidence on the risk factors of occupational exposure to pesticides in
greenhouse work environments?
2) What is the significance of temperature and pesticide formulation factors in determining
the extent of dermal exposure and glove protection performance against OP
insecticides?
Methodology
A combination of a critical literature review and a series of laboratory-based experimental
studies were conducted to answer the above research questions. These are summarised below.
Critical literature review (Study 1)
The critical literature review followed the “Work”, “Worker” and “Workplace” conceptual
framework. The search strategy, the screening, and appraisal processes identified 71 relevant
literature sources for the review. These research articles were categorised into four groups based
on what each studied.
1. Studies that described pesticide exposure scenarios in greenhouses (n=38),
2. Studies that analysed the effect of “Work”, “Worker” and “Workplace” factors on
pesticide exposure (n=20)
3. Studies that investigated pesticide fate and behaviour in greenhouses (n=8)
4. Studies that compared exposure between greenhouses and open farms (n=5) Laboratory-based dermal exposure and glove permeation studies (Studies 2, 3, and 4)
Permeation studies (in vitro human skin and test cell glove permeation) were performed using
OP insecticides selected on the basis of their common use in Australia. For all experiments,
permeation tests involved at least two concentrations of OP insecticides: Spray dilution strength
and mixing concentrate strength. These experiments informed the following three discrete but
related studies.
Study 2: Effect of temperature on comparative protection performance of gloves
- A glove permeation study
- Three glove materials: polyvinylchloride (PVC), nitrile butadiene rubber (NBR), and
nitrile/neoprene (NN)
- Two OP insecticides formulations: dimethoate and malathion
- A range of temperature conditions: 25-60°C.
Study 3: Dermal absorption of OP insecticides as formulations
- In vitro human skin permeation study
- Four OP insecticide formulations: acephate, dimethoate, malathion, and diazinon
- Three skin permeability models (Potts & Guy, Mitragotri, and IHSkinPerm) were
evaluated.
Study 4: The role of formulation co-ingredients in dermal exposure to OP insecticides
- In vitro human skin and glove permeation study
- One glove material: PVC
- Two OP insecticides
- Dimethoate: pure and two commercial formulations (one with xylene and the
other without xylene) Omethoate: pure and two commercial formulations with the main difference
being the presence of the co-ingredient propylene glycol methyl ether acetate
(PGMEA).
- Temperature conditions: 37°C for skin and 35°C for glove experiments
Major findings
This research analysed the risk factors for occupational pesticide exposure in horticulture and
the main findings are broadly discussed in two parts. In part 1, the influence of Work
Environment risk factors such as greenhouse characteristics and temperature on occupational
exposure to pesticides and glove protection performance is discussed. Part 2 outlines how
Chemical risk factors (i.e. physicochemical properties and formulation co-ingredient
compositions of pesticides) influence dermal absorption and glove protection.
PART 1-WORK ENVIRONMENT RISK FACTORS
Greenhouse work environment and pesticide exposure
The critical literature review revealed a number of exposure modifying factors in greenhouses,
which are broadly discussed within the “Work”, “Worker”, and “Workplace” conceptual
framework.
Workplace
The greenhouse work environment is characterized by its enclosed nature, tight space, dense
plant arrangements, and controlled micro-climatic parameters including light, humidity,
temperature, and ventilation. The critical literature review revealed two mechanisms in which
these greenhouse Workplace factors influence pesticide exposure parameters. Firstly,
Workplace factors directly influence pesticide exposure parameters such as the fate,
distribution, and availability of pesticides in greenhouses, bodily distribution of pesticides, and
penetration of pesticides through protective clothes and gloves. Secondly, Workplace factors alter how theWork is done (e.g. type of pesticides used, type of spraying techniques/equipment
used, and the frequency of contact with contaminated surfaces), and how Workers behave (e.g.
potentially poor safety procedures and inadequate PPE use compliance).
Work
The major exposure modifying Work factors in greenhouses were intense workload, quick reentry
interval, and predominantly manual spraying techniques. These were shown to influence
occupational exposure parameters such as the duration, frequency, and route of exposure, the
body part contaminated, and PPE-use compliance.
Worker
The majority of the literature describes greenhouse workers as under-educated, poorly trained,
and socioeconomically disadvantaged migrant workforce with precarious personal
circumstances. Although these Worker characteristics may increase workers’ vulnerability for
pesticide exposure and risk, a systematic understanding of how these characteristics contribute
to pesticide exposure in greenhouses is still lacking.
Overall, the evidence presented in the literature suggests that most of the pesticide exposure
modifying greenhouse characteristics and interactions therein may not be apparent in traditional
open field systems. Therefore, the two environments could have different pesticide exposure
scenarios and may have different health risks as well.
Glove permeation of OP insecticides and temperature
The effect of temperature on permeation resistance of each chemical resistant glove was shown
to be significant, particularly at high exposure concentrations. A comparison of the three gloves
reveals that there was no significant difference in permeation resistance when tested against
dilute forms of dimethoate (0.3 g/l and 1.6 g/l) and Malathion (1 g/l) at 25ºC. However, there
was a significant difference in permeation resistance between glove materials under the “worst-case” exposure scenario of handling highly concentrated formulation products of dimethoate
(400 g/l) and malathion (1150g/l), at elevated temperatures. PVC gloves afforded the highest
permeation resistance, followed by NBR glove and lastly the single-use NN glove material.
PART 2: CHEMICAL RISK FACTORS
Pesticide formulations are often complex mixtures of active insecticides and co-ingredients.
The analysis of Chemical risk factors for occupational exposure to pesticides was possible in
this research because commercially available pesticide formulations were used in skin and
glove permeation experiments. This part was informed by two studies. The first examined skin
permeation of OP insecticides as formulations while the second investigates the role of
formulation co-ingredients on permeation through the skin and PVC gloves.
Skin permeation of OP insecticides as formulations
The purpose of studying skin permeation of OP insecticides as formulations was to determine
the relevance of physicochemical properties and selected Quantitative Structure-Permeability
Relationship (QSPR) models in predicting skin permeability of OP insecticides as formulations.
In terms of physicochemical properties, higher permeation flux was recorded at a higher applied
concentration of OP insecticides for the hydrophilic AI (acephate and dimethoate), but not for
the lipophilic ones (malathion and diazinon). The tested skin permeability models (Potts & Guy,
Mitragotri, and IHSkinPerm) mostly over-predicted experimental outcomes by several orders
of magnitude. Models were much less accurate in predicting permeation from highly
concentrated emulsions of the lipophilic compounds. The model predictions were accurate only
on two out of seven occasions, and only for the hydrophilic OP insecticides.
The role of formulation co-ingredients in dermal exposure to OP insecticides
The findings showed that the presence of xylene in dimethoate formulation and presence of
PGMEA in omethoate formulation plays a critical role in the permeation of the respective OP insecticides through PVC gloves at high mixing concentration. The omethoate formulation
without PGMEA content, showed 184-fold greater cumulative permeation and more than 16
fold shorter breakthrough (BT) time. The presence of xylene in dimethoate formulation
increased the cumulative permeation (by 1.5 fold) and decreased BT time (by an hour) of
dimethoate. The influence of these co-ingredients on skin uptake within the exposure
timeframe was not statistically significant at both high and low OP insecticide concentrations
which may be due to the high degree of variability with skin experiments.
Novelty and strength
In all glove and skin permeation experimental studies included in this thesis, commercially
available pesticide formulations of OP insecticides were employed in the context of their use
in horticulture. Thus, this research is the first to report empirical skin/glove permeation data for
most of the OP insecticides studied. The two major strengths of this research are 1) framing of
experimental studies around real-world exposure scenarios within the context of Australian
horticulture, and 2) the use of fresh human donated skin, which is considered a gold standard
for in vitro dermal absorption studies.
Limitations
This research is largely informed by laboratory-based experiments, hence it may not capture all
the factors that would influence exposure in the field. Despite its limitation, it was supplemented
by a critical review that captured the exposure modifying factors in greenhouses.
Conclusions and recommendations
This thesis combines multiple pieces of evidence from a critical literature review and a series
of laboratory-based experimental studies to extend our knowledge of pesticide exposure risk in
horticulture. Based on the findings the following conclusions can be made: Firstly, the exposure-modifying effect of the greenhouse work environment may set it apart
from traditional open field farming in terms of pesticide exposure and its risks. This necessitates
the need for the development of tailored guidelines for pesticide exposure control in
greenhouses.
Secondly, it was demonstrated that not all recommended gloves are equally protective. The
findings underpin the importance of considering temperature-induced glove performance
reduction in testing, selecting, using, storing, and replacing glove materials if used under hot
working conditions.
Thirdly, the findings from the skin permeation study support the notion that skin permeability
models derived from experimental data on aqueous solutions have limited applicability in
accurately predicting dermal absorption of OP insecticides from commercial formulations.
Fourthly, the findings suggest that the presence and proportions of co-ingredients play a critical
role in glove permeation potential of undiluted OP insecticides, which confirms the relevance
of considering permeation data of co-ingredients in glove selection and recommendation.
Based on the evidence from this research, the following recommendations have been made for
various stakeholders:
- Glove manufacturers should extend standard glove permeation test conditions to
incorporate commercial pesticide formulations, in-use temperatures, and pesticide
concentration conditions.
- Pesticide manufacturers and suppliers should consider the role of formulation coingredients
for dermal exposure when specifying glove materials and when developing
pesticide formulations. Pesticide regulatory agencies should consider the implications of formulation coingredients
in the process of reviewing exposure data as well as making product approval
and registration decisions.
- Horticulture growers and workers handling concentrated products at elevated temperature
conditions, such as in greenhouses, should be aware of the increased exposure risk, the
temperature-induced glove performance reduction, and the need for more frequent
replacement.
- Occupational hygienists and other risk assessors should take caution when using skin
permeability models, developed for aqueous solutions, to predict dermal absorption of
chemicals as formulations, particularly for highly concentrated lipophilic chemicals.
Further research could assess the comparative health risk between greenhouses and open field
horticulture, and the effects of glove reuse and pesticide concentrations on dermal exposure.
Research into the feasibility of co-ingredient modifications to minimise dermal exposure would
also be a fruitful area for further work.
School/Discipline
School of Public Health
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Public Health, 2021
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