Annals of Occupational Hygiene Advance Access originally published online on August 26, 2005
Annals of Occupational Hygiene 2005 49(8):703-709; doi:10.1093/annhyg/mei033
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© 2005 British Occupational Hygiene Society Published by Oxford University Press
Original Article |
Exposure Misclassification of Household Pesticides and Risk Perception and Behaviour
1 Department of Environmental Science and Technology, Imperial College, London, UK; 2 ALSPAC, Bristol University
* Author to whom correspondence should be addressed. Tel: +44 20 7594 6384; fax: +44 20 7594 9266; e-mail: m.nieuwenhuijsen{at}imperial.ac.uk
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODOLOGYRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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The aim of this paper was to compare self-reported household pesticide use or non-use in a questionnaire with reported household pesticide use from an in-depth interview, in order to elucidate any differences, and to study any differential reporting of pesticides. In the in-depth interview we asked for pesticide use, behaviour adopted while using pesticides and risk perceptions as possible factors to explain the reporting of pesticide use. The Avon Longitudinal Study of Parents and Children (ALSPAC) was used as the sampling frame. Eight hundred and thirty one parents filled out and returned the questionnaire. A random sample of 53 users and 94 non-users took part in the interview. Almost 90% of the 94 who did not report the use of pesticides in the questionnaire reported the use of pesticides during the interview. However, those who reported pesticide use in the questionnaire were more likely to report home and garden pesticide use (P < 0.05) in the interview. The parents who reported pesticide use in the questionnaire had a tendency to perceive a lower risk and higher benefit from pesticide use, and tended to be less risk averse when compared with the groups of parents who reported no pesticide use. They bought the pesticides because ‘they looked safe’, while those who did not report pesticide use bought them because they ‘used them before’. The latter were also more likely to state that they did not understand everything on the label and that they thought that it did not provide all the information needed. They were also less likely to feel that they knew what they are doing when using pesticides and felt that pesticide use is relatively dangerous compared with other hazards. In conclusion, pesticide use is underreported in questionnaires, and behaviour and risk perception may affect the reporting.
Keywords: children • exposure misclassification • pesticides • risk perception
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODOLOGYRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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A number of epidemiological studies, mostly conducted in the USA, have associated the use of pesticides in the home and garden with a variety of adverse effects, including childhood cancers (Gold et al., 1979
; Reeves et al., 1981; Lowengart, 1987; Buckley et al., 1989; Davis et al., 1993; Leiss and Savitz, 1995; Pogoda and Preston-Martin, 1997; Meinert et al., 2000; Ma et al., 2002). Underreporting of pesticide usage leads to misclassification of exposure to pesticides within homes and gardens and result in loss of power in epidemiological studies (Armstrong, 2003). If differential, it may bias the risk estimates. In epidemiological studies, exposure is the most neglected and traditionally difficult aspect to be measured with accuracy (Nieuwenhuijsen, 2003). Most of the epidemiological studies available are retrospective and the available pesticide exposure information from home and garden pesticide use is either subject to recall bias by participants or derived from surrogate information. Furthermore, in epidemiological studies only limited information can be asked in questionnaires on pesticide use because of the pressures on time and space and this may contribute to underreporting of pesticides. Attempts to associate pesticide use with childhood diseases are often flawed by lack of accurate exposure information (Daniels et al., 1997). It has been shown that the more detailed the information regarding exposure, the more significant the association between use and adverse health effects (Daniels et al., 1997). There is some evidence of underreporting in the occupational environment (Tielemans et al., 1999; Blair et al., 2002; Engel et al., 2002), but little in the home environment. Also, consumer use and behaviour may vary according to attitude, experience and the understanding of the risks involved (Teuber, 1989) and could affect the reporting of pesticides. This necessitates a better understanding of these factors. The aim of this paper was to compare self-reported household pesticide use or non-use in a questionnaire with reported household pesticide use from an in-depth interview, in order to elucidate any differences, and to study any differential reporting of pesticides. In the interview we asked for pesticide use, behaviour adopted while using pesticides and risk perceptions as possible factors to explain the reporting of pesticide use.
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METHODOLOGY |
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TOPABSTRACTINTRODUCTIONMETHODOLOGYRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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As the sampling frame, we used the Avon Longitudinal Study of Parents and Children (ALSPAC), which examines the relation between environment and development and health of children and their parents. The ALSPAC study has been collecting information on the physical environment on a cohort of
14 000 children, prenatally to the present day (Golding et al., 2001). To be eligible, the expected date of birth had to lie between April 1991 and December 1992 and the mother had to live in the Avon area in and surrounding Bristol (ALSPAC, 2001). Half the population lives in Bristol but the rest were either in suburbs, in rural villages or in small and large towns.
A short questionnaire (similar to the ALSPAC questionnaire on pesticide use) was distributed directly to parents at ALSPAC Focus (examination) Clinics between July and October 2001, which identified their status in the study based on their self-reported pesticide use or non-use and confirmed willingness for further interview of the parents. The parents were asked how often they used (i) weed killers and (ii) pesticides/insect killers (including flea or fly sprays or powders). (For the rest of the paper these two groups were combined and referred to as pesticides.) A total of 831 parents filled out and returned the questionnaire, resulting in a high (96%) return rate. Out of them, 713 (86%) parents were willing to be interviewed further at their homes about their pesticide use, and these were stratified into families that reported any pesticide ‘use’ or ‘non-use’ in the questionnaire. A random sample was chosen for an in-depth interview within these use and non-use strata with an approximate ratio of 1:2, since a pilot study suggested that there might be underreporting of pesticide use due to recall bias. Of the original target of 150, 53 users and 94 non-users were interviewed. Time and resource constraints limited the number of people who could be interviewed for this study. Everyone approached agreed to be interviewed. The in-depth face-to-face interviews took place at the subjects' home in the autumn (October–November). Pesticides recall was for the previous year. Each interview took between 30 and 40 min to complete. Further details can be found in Grey (2003).
During the interview parents were asked in detail about their pesticide use and to express their attitudes and beliefs concerning pesticides, through a series of probing statements. A thorough review had been made of the available literature in order to elucidate the main relevant attitudes to be tested. The interview used a mixture of open-ended questions and six-point Likert-type questioning, where respondents were asked to indicate their degree of agreement or disagreement rated on a response scale of 1–6, where 1 would represent one extreme of the attitude scale and 6 would represent the other extreme (strongly agree = 1, agree = 2, slightly agree = 3, slightly disagree = 4, disagree = 5 and strongly disagree = 6) (Sarantakos, 1994). Similar scales were used for risk (not risky = 1, slightly risky = 2, moderately risky = 3, quite risky = 4, risky = 5 and very risky = 6) and benefits (very high benefit = 1, high benefit = 2, moderate benefit = 3, low benefit = 4, very low benefit = 5 and no benefit = 6). The risk questions and analytical methods were developed through pilot testing and examination of questionnaires used in previous studies (Grieshop and Stiles, 1989; Siegrist and Cvetovich, 2000).
The raw data from the questionnaires and interviews were coded and entered into specially designed Access (Microsoft Access 2000) databases. Once the data had been initially entered it was carefully checked for entry errors. The results were analysed using SPSS 11.0 for windows (SPSS Inc.). Potential predictors of pesticide use were explored using multiple logistic regression analysis.
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RESULTS |
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Demographics
The demographics of the self-reported pesticides users and non-users showed very similar trends (data not shown but available in Grey, 2003
). The largest proportion of participants was between 25 and 35 years of age, whites, non-smokers, and owned their own property. Those who did not report pesticide use were slightly better educated and earned slightly more.
Use and behaviour
All the subjects who reported pesticide use in the questionnaire also reported pesticide use in the interview. Of the 94 who did not report pesticide use in the questionnaire, 80 reported pesticide use in the interview. Table 1 provides a further breakdown of the pesticide use reported during the interview by pesticide use at the questionnaire. In general, a higher percentage of those who reported use in the questionnaire reported the use of various classes of pesticides in the interview compared with those who did not, but the differences was small for lice treatments and garden pesticides and larger for pesticides used on pests and in the home. The differences were statistically significant for home and garden pesticides, but not lice treatment and use on pests.
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Logistic regression analyses showed that self-reported use of pesticides in the questionnaire was significantly associated with the use of both garden pesticides and home pesticides in the interview. Parents who reported using pesticides in the questionnaire were 1.4 times more likely to report garden pesticides during the interview compared with those parents who did not report pesticide use in the questionnaire (OR = 1.45, 95% CI 1.12–1.85). They were also 1.7 times more likely to report the use of pesticides in their homes (OR = 1.70, 95% CI 1.09–2.59), 2.3 times more likely to report the use of weed killers (OR = 2.26, 95% CI 1.12–4.46), 1.9 times more likely to report the use of insecticides in their garden (OR = 1.94, 95% CI 1.17–3.20) and 1.6 times more likely to report the use of insecticides in their homes (OR = 1.61, 95% CI 1.01–2.58).
Table 2 shows the responses of parents when they were asked when they would use pesticides. Those who did not report the use of pesticides in the questionnaire during the interview reported more often that they would not use a pesticide or would use a non-chemical method. Those who reported pesticide use in the questionnaire were 2.5 times more likely to use pesticides against indoor pests at the first sign of a problem compared with those who did not report pesticide use (OR = 2.53, 95% CI 1.17–5.46), less likely to choose to use non-chemical methods as a first line of defence against garden pest (OR = 0.40, 95% CI 0.18–0.90) and less likely to state that they would never use pesticides against garden pests (OR = 0.26, 95% CI 0.10–0.90), and 4.6 times more likely to use pesticides at the first sign of a problem to treat weeds in their garden (OR = 4.60, 95% CI 1.34–15.77).
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Parents were asked whether or not they applied chemical products to treat pests or weeds, and what they generally thought about the pesticide product labels (Table 3). There were no significant differences between the self-reporting of pesticide use or no use in the questionnaire and what the parents thought about the pesticide labels. However, those who did not report pesticide use were more likely to state that they did not understand everything on the label and that they thought that it did not provide all the information needed. When asked why they bought the products, those who did not report pesticide use appeared to buy the pesticides because they ‘used them before’ and those who reported pesticide use because ‘they looked safe’ (Table 4).
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Risk and attitudes
A number of associations could be found when comparing self-reporting of pesticide use or no use in the questionnaire and risk perception and attitudes (Table 5). The parents who reported pesticide use in the questionnaire had a tendency to perceive a lower risk and higher benefit from pesticide use, and tended to be less risk averse when compared with the groups of parents who reported no pesticide use in the screening questionnaire. This is illustrated using the results for the total risk perception.
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The risk statements showed that there are significant differences between self-reported pesticide users and non-users in the questionnaire in terms of the feeling that there is a risk to both themselves and to people in general from all uses of pesticides. A difference was also seen in the risk to pets from the parent's own use of pesticides. For all three statements, the self-reported pesticide non-users appear to feel that there is a higher risk compared with the pesticide users.
There were significant differences between the two groups for all pesticide uses, except for houseflies, when asked about the perceived benefit of using a pesticide to treat specific pests. In all the cases the self-reported pesticide users in the questionnaire appear to feel that there is a higher benefit in using the pesticides, when compared with those who did not report pesticide use.
The attitude statements show that there are significant differences between the two groups for attitudes that examine the importance of voluntariness, experience, trust, and relative risk. Those who did not report pesticide use in the questionnaire disagreed that they are always careful when using pesticides, that they have never experienced problems when using pesticides and that they trusted the regulators to make the use of pesticides safe. They were also less likely to feel that they know what they are doing when using pesticides and felt that pesticides were relatively dangerous compared with other hazards.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODOLOGYRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Almost 90% of those who did not report the use of pesticides in the questionnaire reported the use of pesticides during the interview. However, those who reported pesticide use in the questionnaire were also more likely to report pesticide use during the interview. The differences were relatively small for lice treatments and garden pesticides and larger for pesticide use on pests and in the home. The differences were statistically significant for home and garden pesticides, but not lice treatment and use on pests, suggesting that the former defines the difference between the two groups. The parents who reported pesticide use in the questionnaire had a tendency to perceive a lower risk and higher benefit from pesticide use, and tended to be less risk averse when compared with the groups of parents who reported no pesticide use. They bought the pesticides because ‘they looked safe’, while those who did not report pesticide use bought them because they ‘used them before’. The latter were also more likely to state that they did not understand everything on the label and that they thought that it did not provide all the information needed. They were also less likely to feel that they knew what they are doing when using pesticides and felt that pesticides are relatively dangerous compared with other hazards.
The current ALSPAC study, as many other epidemiological studies, has only a limited number of questions (two in this case) regarding pesticide use inside the home and in the garden (similar to the questionnaire questions), which are being asked on a regular basis. This study suggests that cohort members therefore may underreport pesticide use, although those who report pesticide use are more likely to use them. In this study, the sensitivity and specificity of the questionnaire was 39 and 100%, respectively. The specificity is generally more important in epidemiological studies than the sensitivity, although this study suggest that there are very few non-users (10%) (Armstrong, 2003). This misclassification of exposure to pesticides within homes and gardens will lead to loss of power in an epidemiological study. Furthermore, if differential it may lead to bias in the health risk estimates.
Comparing self-reported pesticide use in the questionnaire with pesticide use derived from the interview suggest that the parents who initially stated in the questionnaire that they were pesticide users may be more aware of their use, or more willing to report them. The question is why this is happening?
First of all, the results suggest that the pesticides that are used in the home and the garden define the difference between the two groups, and they may be perceived as ‘pesticides’. Lice and pet treatments appear not to be perceived to be pesticides. Also, the use of home and garden pesticides was much more frequent compared to lice and pet treatments and therefore may be easier to remember.
Secondly, there are some behavioural differences between the groups. When treating pest and weed problems, those who reported using pesticides in the questionnaire were less likely to choose a non-chemical alternative to pesticides in the garden and less likely to leave the garden pest problem untreated. These parents were also more likely to treat a weed problem preventatively by using pesticides compared with those who had reported no use of pesticides in the questionnaire. Overall, it appears that self-reported users of pesticides from the questionnaire are happier to use pesticides. A study by Grieshop and Stiles (1989) suggested that more experienced pesticide users also showed less self-protective behaviours (Grieshop and Stiles, 1989), probably because people must believe that a threat applies to them before considering preventative behaviours (Neuwirth et al., 2000). In the case of using pesticides inside the home, both groups of parents would not use pesticides preventatively only when they had noticed a pest problem, suggesting that indoor and outdoor environments are perceived differently in terms of pesticide use. No differences were seen in behaviour of self-reported pesticide users and non-users in their application of head lice treatments on their children, where overwhelmingly the majority of both groups of parents preferred to treat head lice using non-chemical methods, specifically using a head lice comb and conditioner method, known as ‘bug busting’.
Thirdly, there was a difference in attitude and risk perception. Those who did not report pesticide use in the questionnaire perceived higher risk and lower benefit from pesticide use, and tended to be more risk averse when compared with the groups of parents who self-reported of using pesticide. Slovic et al. (1980) found that imaginability is likely to influence risk attitudes and behaviours, where frequent use of pesticides with no precautions and no ill effects can reduce the ability to imagine about its harm. Self-reported pesticide users in the questionnaire tended to perceive a greater risk to pets from their own pesticide use, as well as perceive a greater risk from general pesticides compared with those they would apply themselves. This is likely to be caused by a feeling of voluntariness, where the risk to a pet and the risk from all use of pesticides were felt to be less voluntary, and therefore perceived as greater risks.
Those who did not report pesticide use in the questionnaire perceived a lower benefit from using pesticides to treat pest problems. Generally, inverse relationships between judged risks and benefits have been observed in various studies (Alhakami and Slovic, 1994; Frewer et al., 1998; Grey 2003), and in this case it would seem likely that the parents who reported that they did not use pesticides felt that pesticide were of a low benefit because they perceived pesticides as being risky. Those who did not report pesticide use in the questionnaire tended to differ from those who reported pesticide use in terms of the attitudes that drive their perception of pesticide risk. These included voluntariness, experience, trust and relative risk. They stated that they were not always careful when using pesticides, were more likely to have experienced problems with pesticides, less likely to trust regulators to make the use of pesticides safe, and felt that pesticides were relatively dangerous compared with other hazards. This suggests that these attitudes are important factors affecting both the perceived risk of pesticides and the reporting of pesticide use. They also underreported their pesticide use.
Implications
A few epidemiological studies have suggested an association between pesticide use in the home and garden with adverse health effects in children (Gold et al., 1979; Reeves et al., 1981; Lowengart, 1987; Buckley et al., 1989; Davis et al., 1993; Leiss and Savitz, 1995; Pogoda and Preston-Martin, 1997; Meinert et al., 2000; Ma et al., 2002). However, the exposure classification tends to be crude and based on retrospective use of pesticides recalled by parents, which increases the risk that any associations will be diluted and go undetected (Zahm and Ward, 1998). The findings of this pesticide study would suggest that any measure of pesticide exposure based on the crude recall of pesticide use by amateur users will not give an accurate depiction of the pesticide products used, as a result of underreporting. This shows how difficult it will be to accurately measure pesticide use retrospectively using a simple questionnaire. In order to get more accurate measures of exposure it is necessary to use more in-depth techniques such as an in-depth interview where pesticide use can be prompted in order to aid recall, which has been proven by this study to be much more successful. Furthermore, this study shows that risk perception, attitude and behaviour may differ by pesticide use reporting in a short questionnaire, raising the question whether there may be also effects on other factors, including lifestyle, which may be confounding any potential association between exposure to pesticides and adverse health effects.
The current research implies that we may need to add questions on risk perception and attitude in the exposure questionnaire to capture factors that may lead to any potential underreporting. This information could then be used to model ‘true’ pesticide use. However, few studies have been conducted in this area and further research is needed to confirm or refute these findings, and also to estimate the contribution. If there is only a minor contribution of these factors to the reporting then it may not warrant a large section in an exposure questionnaire. However, it may if it appears to make a large contribution. The relationship between exposure prevalence, risk perception and attitude, confounders and health effects is likely to be complex and may vary by situation. It requires a carefully thought out research approach to disentangle the various issues but it may be worth while in times when issues such as risk perception and health effects appear more and more often together.
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ACKNOWLEDGEMENTS |
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The authors are extremely grateful to all the mothers who took part and to the midwives for their cooperation and help in recruitment. The whole ALSPAC Study Team comprises interviewers, computer technicians, laboratory technicians, clerical workers, research scientists, volunteers and managers, who continue to make the study possible. This study could not have been undertaken without the financial support of the Medical Research Council, the Wellcome Trust, UK government departments, medical charities and others. The ALSPAC study is part of the WHO initiated European Longitudinal Study of Pregnancy & Childhood.
Received February 2, 2005; in final form June 20, 2005
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REFERENCES |
|---|
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TOPABSTRACTINTRODUCTIONMETHODOLOGYRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Alhakami A, Slovic P. (1994). A psychological study of the inverse relationship between perceived risk and perceived benefit. Risk Anal; 14: 1085–96.[CrossRef][Web of Science][Medline]
ALSPAC, 2001. Available on http://www.ich.bris.ac.uk/alspacext/Default.html (accessed June 2001).
Armstrong B. (2003) Exposure measurement error: consequences and design issues. In Nieuwenhuijsen MJ, editor. Exposure assessment in occupational and environmental epidemiology. Oxford: Oxford University Press.
Blair A, Tarone R, Sandler D et al. (2002) Reliability of reporting on life-style and agricultural factors by a sample of participants in the Agricultural Health Study from Iowa. Epidemiology; 13: 94–9.[CrossRef][Web of Science][Medline]
Buckley JD, Robison LL, Swotinsky R et al. (1989) Occupational exposures of parents of children with acute nonlymphocytic leukemia: a report from the Childrens Cancer Study Group. Cancer Res; 49: 4030–7.
Daniels J, Olsham A, Savitz D. (1997) Pesticides and childhood cancers. Environ Health Perspect; 105: 1068–77.[Web of Science][Medline]
Davis J, Brownson R, Garcia R et al. (1993) Family pesticide use and childhood brain cancer. Arch Environ Contam Toxicol; 24: 87–92.[CrossRef][Web of Science][Medline]
Engel LS, Seixas NS, Keifer MC et al. (2001) Validity study of self-reported pesticide exposure among orchardist. J Expo Anal Environ Epidemiol; 11: 359–68.[CrossRef][Web of Science][Medline]
Frewer LJ, Howard C, Hedderley D et al. (1998) Methodological approaches to assessing risk perceptions associated with food-related hazards. Risk Anal; 18: 95–102.[CrossRef][Web of Science][Medline]
Gold E, Gordis L, Tonasica J et al. (1979) Risk factors for brain tumours in children. Am J Epidemiol; 109: 309–19.
Golding J, Pembrey M, Jones R et al. 2001. ALSPAC—The Avon Longitudinal Study of Parents and Children 1. Study Methodology. Paediatr Perinat Epidemiol; 15: 74–87.[CrossRef][Web of Science][Medline]
Grey CNB (2003) Household Pesticides in the UK: Use, Risk Perception and Policy Implications, PhD thesis, University of London.
Grieshop J, Stiles M. (1989) Risk and home pesticide users. Environ Behav; 21: 699–716.[Abstract]
Leiss J, Savitz D. (1995) Home pesticide use and childhood cancer: a case-control study. Am J Public Health; 85: 249–51.
Lowengart RA, Peters JM, Cicioni C. (1987). Childhood leukaemia and parent's occupational and home exposures. J Natl Cancer Inst; 79: 39–46.[Web of Science][Medline]
Ma X, Buffler PA, Gunier RB et al. (2002) Critical windows of exposure to household pesticides and risk of childhood leukemia. Environ Health Perspect; 110: 955–60.[Web of Science][Medline]
Meinert R, Schuz J, Kaletsch U et al. (2000) Leukemia and non-Hodgkin's lymphoma in childhood and exposure to pesticides: results of a register-based case-control study in Germany. Am J Epidemiol; 151: 639–46; discussion 647–50.
Neuwirth K, Dunwoody S, Griffin R. (2000) Protection motivation and risk communication. Risk Anal; 20: 721–34.[CrossRef][Web of Science][Medline]
Nieuwenhuijsen MJ. (2003) Exposure assessment in occupational and environmental epidemiology. Oxford: Oxford University Press.
Pogoda J, Preston-Martin S. (1997) Household Pesticides and the Risk of Paediatric Brain Tumours. Environ Health Perspect; 105: 1214–20.[Web of Science][Medline]
Reeves JD, Driggers DA, Kiley VA. (1981) Household insecticide associated aplastic anaemia and acute leukaemia in children. Lancet; 2: 300–1.[CrossRef][Web of Science][Medline]
Sarantakos S. (1994) Social Research. ISBN 0-333-60732-5 The Macmillan Press Ltd, London.
Siegrist M, Cvetovich G. (2000). Perception of hazards: the role of social trust and knowledge. Risk Anal; 20: 713–9.[CrossRef][Web of Science][Medline]
Slovic P, Fischoff B, Lichtenstein S. (1980) Facts and fears: understanding perceived risk. In Schwing RC, Albers WA Jr, editors. Societal risk assessment: how safe is safe enough? New York: Plenum Press.
Teuber A. (1989) Justifying risk. Daedalus; 119: 235–252.
Tielemans E, Heederik D, Burdorf A et al. (1999) Assessment of occupational exposures in a general population: comparisons of different methods. Occup Environ Med; 56, 145–51.
Zalm S, Ward M. (1998) Pesticides and childhood cancer. Environ Health Perspect; 106: 893–908.
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