The literature review shows that the amount of work published on glyphosate levels in the population continues to be very limited. In the last 2 years, only five new studies reporting actual values of glyphosate in humans were published, and even when combined with the studies in our previous review, only 4299 individuals have been tested worldwide [6, 8, 9, 11,12,13,14,15] for their urinary glyphosate level and only 520 of them are children [8, 9, 11, 12]. Only two of the new studies were conducted in the United States [11, 13], bringing the total number of studies conducted in the US to six. Another remarkable aspect of this review is the paucity of measurements of AMPA together with glyphosate. Measuring residues and metabolites allow to better understand the individual ability to degrade the main compound, as well as to detect other by-products, such as AMPA, which carries its own safety concerns [16]. Additionally, since glyphosate is non-persistent with an estimated biological half-life in urine ranging between five and a half and 10 h depending on the measurement and adjustment methods [17], measurement of AMPA levels within these samples is essential for accurate estimation of true exposure levels.
In terms of laboratory methods, fluorescent covalent microbead immunoassay [8], ELISA [9], HPLC [14], LC-MS/MS [13], HPLC-MS/MS [11, 12],and 9-fluorenylmethyloxycarbonyl chloride to derivatize urine samples prior to using high pressure liquid chromatography were deployed [15]. Although there is not a definitive best practice for measurement of glyphosate levels. Research with other non-persistent chemicals with short half-lives like bisphenol A has found that ELISA lacks the necessary sensitivity and specificity to accurately measure exposure [18]. MS provides a low LOD and is routinely used by the CDC’s Environmental Health Laboratory [19], suggesting it is likely a more robust measurement method.
When reviewing the published literature, we endeavored to find statistical methods for measuring the accuracy or at least consistency of the results reported. Most of the studies, however, did not report repeated measures that would allow for calculation of coefficient of variation or comparison across methods. Researchers might consider reporting the coefficient of variation or intra-correlation coefficient within the results to allow for better comparison between studies.
Of interest, there are now six data sets from four distinct studies reporting on children. All the studies confirm the presence of glyphosate in urine samples from children, both within and outside of agricultural communities, with values exceeding those measured in adults when the corresponding values were available. Previous research suggests multifactorial reasons for higher levels of environmental toxin exposure in children, which may include smaller body mass or higher likelihood of ingestion, particularly in younger children who are inclined to put non-food items in their mouths. Children may also be exposed through their play on the ground and in the dirt, particularly dirt from playgrounds, which may be contaminated by toxins deposited in the soil or dust. This may be particularly true in households where parents are occupationally exposed to toxins and may bring those toxins home inadvertently on their clothing, thereby increasing their child’s exposure. Additionally, even if children’s exposure levels are not higher than adults, their growth and maturation may be impacted by toxins in a way that adults’ will not be. Furthermore, they may be more likely to develop disease from exposure given the increased number of years for chronic exposure a child might have as compared to someone exposed in adulthood [7]. Given the evidence that children are especially vulnerable to environmental carcinogens [20, 21], tracking exposure to products such as glyphosate in children is a pressing public health priority. The lack of data on glyphosate exposure in children, and on time trends, geographic variability, and sources of exposure call for systematic monitoring of glyphosate and for more studies on the biological effects of the exposure in the general population as well as in vulnerable subsets.
The levels of glyphosate exposure in occupationally exposed individuals is also a major concern. Although the highest average urinary level of glyphosate reported in these updated studies (AM [range]: 4.0 [1.3–12.0] μg/L) [13] did not reach the highest level seen in previous studies we reviewed, (AM [range]:73.6 [40.2- > 80.0] μg/L) [22], the average urinary glyphosate level in occupationally exposed individuals is still disconcertingly high. The decrease in average levels may reflect better personal protective equipment usage, in light of the increased publicity surrounding the potential hazards of glyphosate exposure. However, given the results of a 2017 study showing glyphosate and AMPA levels increasing over time in non-farmer US and European adults [23], it the current result is likely a reflection of a smaller number of studies, only one of which was conducted in the US, published in the last few years compared to the last review we published.
In terms of methodological aspects, of the three studies that measured occupational exposure, two collected convenience samples from farmers, rather than collecting samples at set intervals before and after exposure. None of the studies collected information on urinary glyphosate levels prior to occupational use. This, combined with the lack of data on glyphosate levels in the non-occupationally exposed general population, makes parsing out the various sources of the observed glyphosate levels in the occupationally exposed difficult, as such levels can also derive from food, drinking water, wind or dust in addition to occupational exposure [24,25,26]. We suggest that studies measuring glyphosate levels in occupationally exposed individuals should attempt to collect samples prior to application, the day of application, and in the days after application, as well as gathering information about how regularly farmers apply glyphosate. This will provide sufficient information to distinguish a baseline level of exposure from the effect of recent application. Regardless, without more standardized measurement practices, it remains difficult to establish just how much more exposure occupationally exposed individuals have compared to the average adult.
Monitoring of urinary glyphosate levels should be conducted in the general population, but is especially important for those who are occupationally exposed and those vulnerable. We continue to suggest that inclusion of glyphosate as a measured exposure in nationally representative studies like the National Health and Nutrition Examination Survey will allow for a better understanding of the risks that glyphosate may pose and allow for better monitoring of those who are most likely to be exposed and those who are more susceptible to the exposure.