A related anaerobic process is nitrate-dependent iron oxidation; a recent review has highlighted, in the context of this process, how the simultaneous presence of nitrate-reducing and iron-reducing areas can potentially be important to nitrogen cycling.Under anaerobic conditions, iron can also be linked to ammonium oxidation.If reactions that generate N2O are active in any of the above processes, they may be stimulated or suppressed by different forms of iron, such as the two indices examined in this study.The degree of this influence under different conditions will then determine the importance of iron relative to other soil properties.Our treatments consisted of two contrasting values for soil moisture and addition of amendments.This was done in order to explore the importance of iron across a wide range of conditions while at the same time avoiding a cumbersome dataset.It is clear from Figure 1 that the importance of iron can change between the two limits of each treatment variable.For example, between 50 and 100% WHC under ammonium fertilization, iron moves from a position of modest relevance to become the highest-ranked driver.Since our results show the importance of iron only at two distinct values, we do not know how its importance under intermediate conditions changes between the two end values.Even without such intermediate data, the differences between contrasting treatments can aid in understanding the mechanisms at work in generating N2O.In the above example, the importance of iron rises markedly under ammonium fertilization as soil moisture increases from 50 to 100% WHC; FeP surpasses FeA in strength as well.
As mentioned earlier, ammonia is oxidized to hydroxylamine, and this can react with iron to produce N2O.In a wetter soil,dutch buckets solutes are more mobile, which can lead to greater production of hydroxylamine as well as greater contact of hydroxylamine with iron.FeP is also likely to be more soluble than FeA.Any combination of these effects might elevate the importance of iron and change which form is more relevant in explaining the associated N2O data.The overall position of iron among other drivers of N2O emission is determined by both its reactivity and the presence of processes subject to its influence.Ample opportunity for inquiry exists for defining the extent of the relationship between iron and N2O in managed as well as unmanaged ecosystems, and this can provide useful practical and theoretical information.For example, including iron in current models of N2O emission may strengthen their predictive ability.In addition, inasmuch as certain indices of iron can be related to its physical or chemical characteristics, observing the relationship between a given index and N2O production, and how this changes under different conditions, may provide insight into the specific reactions at work.As stated earlier, production of N2O is generally accepted to be a microbial affair, and it is logical to assume that the factors that regulate the activity of N2O-producing microorganisms should be the same factors that regulate N2O production.This is not incorrect, but is perhaps a somewhat restrictive rendering; a more accurate framework might include ‘‘biotic-abiotic reaction sequences’’that generate N2O, such as those outlined above.Indeed, ‘‘the complex interactions that occur between microorganisms and other biotic and abiotic factors’’ have been suggested to be a key part of further understanding greenhouse gas production and improving predictions.Pesticide drift, which is the off-target movement of pesticides, is recognized as a major cause of pesticide exposure affecting people as well as wildlife and the environment.In the United States in 2004, > 1,700 investigations were conducted in 40 states because of drift complaints, and 71% of the incident investigations confirmed that drift arose from pesticide applications to agricultural crops.Pesticide drift has been reported to account for 37–68% of pesticide illnesses among U.S.agricultural workers [California Department of Pesticide Regulation 2008; Calvert et al.2008].Community residents, particularly in agricultural areas, are also at risk of exposure to pesticide drift from nearby fields.
Alarcon et al.reported that 31% of acute pesticide illnesses that occurred at U.S.schools were attributed to drift exposure.The occurrence and extent of pesticide drift are affected by many factors, such as the nature of the pesticide 2010], equipment and application techniques , the amount of pesticides applied, weather , and operator care.Pesticide applicators are required to use necessary preventive measures and to comply with label requirements to minimize pesticide drift.Pesticide regulations such as the Federal Insecticide, Fungicide, and Rodenticide Act and EPA’s Worker Protection Standard require safety measures for minimizing the risk of pesticide exposure , and many states have additional regulations for drift mitigation.Better understanding about the magnitude, trend, and characteristics of pesticide poisoning from drift exposure of agricultural pesticides would assist regulatory authorities with regulatory, enforcement, and education efforts.The purpose of this study was to estimate the magnitude and incidence of acute pesticide poisoning associated with pesticide drift from outdoor agricultural applications in the United States during 1998–2006 and to describe the exposure and illness characteristics of pesticide poisoning cases arising from off-target drift.We also examined factors associated with illness severity and large events that involved five or more cases.Participating surveillance programs identify cases from multiple sources, including health care providers, poison control centers, workers’ compensation claims, and state or local government agencies.They collect information on the pesticide exposure incident through investigation, interview, and medical record review.In California, on some occasions, such as large drift events, active surveillance is undertaken for further case finding by interviewing individuals living or working within the vicinity affected by the off target drift.Although the SENSOR-Pesticides program focuses primarily on occupational pesticide poisoning surveillance, all of the SENSOR-Pesticides state programs except California collect data on both occupational and nonoccupational cases.In California, PISP captures both occupational and nonoccupational cases.SENSOR Pesticides and PISP classify cases based on the strength of evidence for pesticide exposure, health effects, and the known toxicology of the pesticide and use slightly different criteria for case classification categories.This study restricted the analyses to cases classified as definite, probable, possible, or suspicious by SENSOR-Pesticides and definite, probable, or possible by PISP.
We also performed analyses restricted to definite and probable cases only.Because the findings from these restricted analyses were similar to those that included all four classification categories , only the findings that used the four classification categories are reported here.In this study, a drift case was defined as acute health effects in a person exposed to pesticide drift from an outdoor agricultural application.Drift exposure included any of the following pesticide exposures outside their intended area of application: a) spray, mist, fumes, or odor during application; b) volatilization, odor from a previously treated field, or migration of contaminated dust; and c) residue left by offsite movement.Our drift definition is broader than U.S.EPA’s “spray or dust drift” definition, which excludes post application drift caused by erosion, migration, volatility, or windblown soil particles.A drift event was defined as an incident where one or more drift cases experienced drift exposure from a particular source.Both occupational and nonoccupational cases were included.An occupational case was defined as an individual exposed while at work.Among occupational cases, agricultural workers were identified using 1990 and 2002 Census Industry Codes : 1990 CICs, 010, 011, 030; 2002 CICs, 0170, 0180, 0290.Figure 1 presents the process of case selection.We selected cases if exposed to pesticides applied for agricultural use including farm, nursery, or animal production, and excluded cases exposed by ingestion, direct spray, spill, or other direct exposure.We then manually reviewed all case reports and excluded persons exposed to pesticides used for indoor applications , persons exposed within a treated area , and persons exposed to pesticides being mixed, loaded, or transported.Drift cases therefore represented the remaining 9% and 27% of all pesticide illness cases identified by the SENSOR-Pesticides and PISP, respectively.We also searched for duplicates from the two programs identifying California cases.Because personal identifiers were unavailable, date of exposure, age, sex, active ingredients, and county were used for comparison.A total of 60 events and 171 cases were identified by both California programs.These were counted only once and were included only in the PISP total.Drift events and cases were analyzed by the following variables: state, year, and month of exposure, age, sex, location of exposure, health effects, illness severity,grow bucket pesticide functional and chemical class, active ingredient, target of application, application equipment, detection of violations, and factors contributing to the drift incident.U.S.EPA toxicity categories ranging from toxicity I to IV were assigned to each product.
Cases exposed to multiple products were assigned to the toxicity category of the most toxic pesticide they were exposed to.Illness severity was categorized into low, moderate, and high using criteria developed by the SENSOR Pesticides program.Low severity refers to mild illnesses that generally resolve without treatment.Moderate severity refers to illnesses that are usually systemic and require medical treatment.High severity refers to life-threatening or serious health effects that may result in permanent impairment or disability.Contributing factors were retrospectively coded with available narrative descriptions.One NIOSH researcher initially coded contributing factors for all cases.Next, for SENSOR-Pesticides cases, state health department staff reviewed the codes and edited them as necessary.Any discrepancies were resolved by a second NIOSH researcher.For PISP cases, relatively detailed narrative descriptions were available for all incidents.These narratives summarize investigation reports provided by county agriculture commissioners, who investigate all suspected pesticide poisoning cases reported in their county.After initial coding, the two NIOSH researchers discussed those narratives that lacked clarity to reach consensus.Data analysis was performed with SAS software.Descriptive statistics were used to characterize drift events and cases.Incidence rates were calculated by geographic region, year, sex, and age group.The numerator represented the total number of respective cases in 1998–2006.Denominators were generated using the Current Population Survey micro-data files for the relevant years.For total and nonoccupational rates, the denominators were calculated by summing the annual average population estimates.A nonoccupational rate for agriculture-intensive areas was calculated by selecting the five counties in California where the largest amounts of pesticides were applied in 2008.For occupational rates, the denominators were calculated by summing the annual employment estimates including both “employed at work” and “employed but absent.” The denominator for agricultural workers was obtained using the same 1990 and 2002 CICs used to define agricultural worker cases.Moreover, in California, where data on pesticide use are available, incidence was calculated per number of agricultural applications and amount of pesticide active ingredient applied.Incidence trend over time was examined by fitting a Poisson regression model of rate on year and deriving the regression coefficient and its 95% confidence interval.Drift events were dichotomized by the size of events into small events involving < 5 cases and large events involving ≥ 5 cases.This cut point was based on one of the criteria used by the CDPR to prioritize event investigations.Illness severity was dichotomized as low and moderate/high.Simple and multi-variable logistic regressions were performed.Odds ratios and 95% CIs were calculated.To our knowledge, this is the first comprehensive report of drift-related pesticide poisoning in the United States.We identified 643 events involving 2,945 illness cases associated with pesticide drift from outdoor agricultural applications during 1998–2006.Pesticide drift included pesticide spray, mist, fume, contaminated dust, volatiles, and odor that moved away from the application site during or after the application.
Although the incidence for cases involved in small drift events tended to decrease over time, the overall incidence maintained a consistent pattern chiefly driven by large drift events.Large drift events were commonly associated with soil fumigations.Occupational exposure.Occupational pesticide poisoning is estimated at 12–21 per million U.S.workers per year.Compared with those estimates, our estimated incidence of 2.89 per million worker-years suggests that 14–24% of occupational pesticide poisoning may be attributed to off-target drift from agricultural applications.Our study included pesticide drift from outdoor applications only and excluded workers exposed within the application area.Our findings show that the risk of illness resulting from drift exposure is largely borne by agricultural workers, and the incidence was 145 times greater than that for all other workers.Current regulations require agricultural employers to protect workers from exposure to agricultural pesticides, and pesticide product labels instruct applicators to avoid allowing contact with humans directly or through drift.Our study found that the incidence of drift-related pesticide poisoning was higher among female and younger agricultural workers and in western states.These groups were previously found to have a higher incidence of pesticide poisoning.It is not known why the incidence is higher among female and younger agricultural workers, but hypotheses include that these groups are at greater risk of exposure, that they are more susceptible to pesticide toxicity, or that they are more likely to report exposure and illness or seek medical attention.However, we did not observe consistent patterns among workers in other occupations.This finding requires further research to identify the explanation.