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Background Organophosphates are insecticides that inhibit the enzymatic activity of acetylcholinesterase (AChE). Because of this, AChE is considered a physiological marker of organophosphate exposure in agricultural settings. However, limited research exists on the associations between urinary organophosphate metabolites and AChE activity in children. Methods This study included 526 participants from 2 exams (April and July–October 2016) of ages 12–17 years living in agricultural communities in Ecuador. AChE activity was measured at both examinations, and organophosphate metabolites, including para-nitrophenol (PNP), 3,5,6-trichloro-2-pyridinol (TCPy), and malathion dicarboxylic acid (MDA) were measured in urine collected in July–October. We used generalized estimating equation generalized linear model (GEEGLM), adjusting for hemoglobin, creatinine, and other demographic and anthropometric covariates, to estimate associations of urinary metabolite concentrations with AChE activity (July–October) and AChE% change between April and July–October. Results The mean (SD) of AChE and AChE% change (April vs July–October) were 3.67 U/mL (0.54) and −2.5% (15.4%), respectively. AChE activity was inversely associated with PNP concentration, whereas AChE% change was inversely associated with PNP and MDA. There was evidence of a threshold: difference was only significant above the 80th percentile of PNP concentration (AChE difference per SD increase of metabolite = −0.12 U/mL [95%CI: 0.20, −0.04]). Likewise, associations with AChE% change were significant only above the 80th percentile of TCPy (AChE % change per SD increase of metabolite = −1.38% [95%CI: 2.43%, −0.32%]) and PNP -2.47% [95%CI: 4.45%, −0.50%]). PNP concentration at ≥80th percentile was associated with elevated ORs for low AChE activity of 2.9 (95% CI: 1.5, 5.7) and for AChE inhibition of ≤ -10% of 3.7 (95% CI: 1.4, 9.8). Conclusions Urinary organophosphate metabolites, including PNP, TCPy and MDA, particularly at concentrations above the 80th percentile, were associated with lower AChE activity among adolescents. These findings bring attention to the value of using multiple constructs of pesticide exposure in epidemiologic studies.

Although Spain does not have a regular national human biomonitoring program yet, different research groups are active in evaluating the exposure of children and adults to chemicals. In the last seven years, several studies in Spain have evaluated the internal exposure of the population to currently used pesticides. The present review analyzes the scope of these studies, the employed analytical methods and the main results of the exposure and risk, mainly for children and mothers. The frequency of exposure to biomarkers of exposure to organophosphate pesticides is high. Some non-specific dialkyl phosphate metabolites (DAPs), such as the diethyl phosphate (DEP), present Detection Frequencies (DFs) in the range of 65–92% in various studies. Also, the specific biomarker of the chlorpyrifos (3,5,6-trichloro-2-pyridinol, TCPy), achieves Detection Frequencies between 74% and 100% in many studies. For pyrethroids, the metabolite 3-phenoxybenzoic acid (PBA) is present, in general, in more than the 65% of the studied samples. Highly polar herbicides were only assessed in one study and both glyphosate and its metabolite aminomethylphosphonic acid showed Detection Frequencies around 60%. However, putting the biomonitoring data in a risk assessment context, the mean Hazard Quotient (HQ), used as a metric for the individual risk, ranges from 0.0006 (glyphosate) to 0.93 in farm workers (parathion), which means that is unlike that the exposure poses a health concern (HQ