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Chlorpyrifos (sometimes referred to as chlorpyrifos-ethyl) and chlorpyrifos-methyl are chlorinated organophosphorus ester pesticides (insecticides, acaricides and miticides) among the most commonly used in Europe. They are used to control insect pests on a range of crops, and chlorpyrifos-methyl is also used to treat stored cereal grain and empty warehouses. Their residues are often present in fruits, vegetables, cereals and dairy products, as well as in drinking water. In the European Union, chlorpyriphos is classified according to the REACH Regulation on harmonised classification and labelling1 of the European Chemicals Agency (ECHA) as “very toxic to aquatic life with long lasting effects and may cause an allergic skin reaction2” . By mid-2019, eight countries had already banned the use of the product for agricultural uses.
1 www.echa.europa.eu/information-on-chemicals
2 https://echa.europa.eu/substance-information/-/substanceinfo/100.024.556
In the E.U., the approval period for chlorpyrifos expires in January 2020. This active substance is covered by the third batch of the renewal programme for pesticides ('AIR3') in accordance with EU Commission Implementing Regulation No 844/2012 and thus under revision for inclusion in Annex I to E.U. directive 91/414/CEE3.
In this context, the European Commission asked the European Food Safety Authority (EFSA) to provide a statement on the available results of the human health assessment and to indicate whether the active substances chlorpyrifos and chlorpyrifos-methyl can be expected to meet the approval criteria which are applicable to human health as laid down in EC Regulation No 1107/20094.
In April 2019, as part of the standard regulatory renewal of approval processes for these substances, experts from EFSA and Member States concluded that concerns related to human health exist.
For chlorpyriphos, EFSA has identified concerns about possible genotoxic effects as well as neurological effects during development, supported by epidemiological data indicating effects in children. This means that no safe exposure level – or toxicological reference value – can be set for the substance5.
For chlorpyrifos-methyl, applications for the renewal of approval were submitted and an initial evaluation of the dossiers was provided to EFSA by the rapporteur Member State (Spain) in a Renewal Assessment Report in July 2017.
Subsequently, EFSA initiated a peer review of the pesticides risk assessment in line with the provisions of Commission Implementing Regulation (EU) No 844/2012. In July 2019, prior to completion of a peer review process and to take into account the comments submitted by the applicants on the previous statement, EFSA organized a further expert meeting.
This means that, once the European Commission has formally adopted the Regulations (expected in January 2020), Member States must withdraw all authorisations for plant protection products containing the active substances. A short period of grace for final storage, disposal and use (maximum 3 months) may be granted by EU countries. After that, such plant protection products can no longer be placed on the market or used in the EU. The Commission is already discussing with the Member States a draft Regulation to lower the Maximum Residue Levels (MRLs) of chlorpyrifos and chlorpyrifos-methyl in food and feed to the lowest level that can be measured by analytical laboratories.
3 Regulation (EU) No 844/2012 of 18 September 2012 setting out the provisions necessary for the implementation of the renewal procedure for active substances, as provided for in Regulation (EC) No 1107/2009 of the European Parliament and of the Council concerning the placing of plant protection products on the market. OJ L 252, 19.9.2012, p. 26.
4 As laid down in Article 4 of Regulation (EC) No 1107/200922 Regulation (EC) No 1107/2009 of the European Parliament and of the Council of 21 October 2009 concerning the placing of plant protection products on the market and repealing Council Directives 79/117/EEC and 91/414/EEC. OJ L 309, 24.11.2009, p. 1.
5 Statement on the available outcomes of the human health assessment in the context of the pesticides peer review of the active substance chlorpyrifos
www.efsa.europa.eu/en/efsajournal/pub/5809
6 www.efsa.europa.eu/en/press/news/chlorpyrifos-assessment-identifies-human-health-effects
7 www.efsa.europa.eu/en/efsajournal/pub/5908
8 https://ec.europa.eu/food/plant/pesticides/approval_active_substances/chlorpyrifos_chlorpyrifos-methyl_en
Based on the above, it is considered that the approval criteria9 for chlorpyrifos-methyl which are applicable to human health are not met and this issue represents a critical area of concern.
9 as laid down in Article 4 of Regulation (EC) No 1107/2009
The approach taken by the experts for the hazard assessment10 of chlorpyrifos-methyl was discussed in the Pesticides Peer Review Experts’ meeting. Initially largely based on the structural similarity with chlorpyrifos, it was eventually recognised that in chlorpyrifos‐methyl, although the chemical structure is similar to chlorpyrifos, some differences have uneven consequences leading to variations which could contribute to the toxicity differences between the two compounds.
In rats, chlorpyrifos‐methyl is extensively absorbed after oral administration, it is widely distributed, extensively metabolized through de‐methylation, hydrolysis and conjugation, then eliminated mostly through urine within 72h. An in vitro metabolism study indicates that the metabolic profiles in rat and human are qualitatively similar, but that chlorpyrifos‐methyl metabolism rate in humans is lower compared to that of rats.
Besides their similar toxicokinetics, chlorpyrifos‐methyl has a higher LD50 than chlorpyrifos, chlorpyrifos‐methyl being ten times less potent upon short‐term exposure than chlorpyrifos in rats and dogs but both having the same level of toxicity upon long‐term exposure.
A problem is that since most impurities were not tested at the levels mentioned in the technical specifications of the substance placed on the market by either of the two applicants, these are not supported by the toxicological assessment. However, regarding the two toxicologically relevant impurities present in the technical specification, it is not expected that they would have the potential to add additional hazard established for the parent compound. The maximum levels of these impurities in the newly proposed technical specifications are in agreement with these requirements.
10 Hazard assessment is the identification of the intrinsic properties of the agent considered To understand more clearly the essential differences between the notions of hazard, risk and safety, watch the short GreenFacts animation video
a) Acute toxicity and metabolic profile
Chlorpyrifos-methyl showed low toxicity when administered by the oral, dermal or inhalation routes. In rats, chlorpyrifos-methyl is extensively absorbed after oral administration, it is widely distributed, extensively metabolised and eliminated mostly through urine within 72h.
The metabolic profiles in rat and human are qualitatively similar, but different in quantitative terms. The substance did not elicit a potential for skin or eye irritation, or for phototoxicity, but was shown to be a skin sensitiser and is classified according to the Classification, Labelling and Packaging (CLP) criteria as skin sensitizer ‘may cause an allergic skin reaction’, as established in the EC Regulation 1272/2008.
b) Chronic toxicity
In all studies conducted with rats, mice and dogs, the main effect following short- to long-term repeated oral administration of chlorpyrifos-methyl was the inhibition of acetyl-cholinesterase (AchE) activity in erythrocytes (red blood cells or RBC), which, at high dose levels, was leading to typical cholinergic symptoms over time. In addition to this effect observed after both chlorpyrifos or chlorpyrifos‐methyl administration, chlorpyrifos‐methyl presented additional critical effects on the adrenals identified as a target organ in short‐term and long‐term toxicity studies.
On this basis, the experts suggested that classification of chlorpyrifos-methyl as acute neurotoxicant, in accordance with the criteria set out in Regulation (EC) No 1272/200811, would be appropriate.
c) Genotoxicity and carcinogenicity
It was noted that there is no public literature available for chlorpyrifos-methyl with regard to the genotoxic potential, while several publications were available for chlorpyrifos. Regarding the molecular structure, the experts considered that the differences between chlorpyrifos and chlorpyrifos‐methyl (the presence of the ethyl group instead of the methyl) would not justify a difference in the genotoxic potential between the two molecules.
The majority of experts considered that the public literature indications, although presenting some limitations (e.g. literature search methodology, no guideline compliant studies, no data reported for positive controls, etc.), should be considered in a weight‐of‐evidence approach and raised concerns over the potential for DNA damage for chlorpyrifos‐methyl, by adopting a conservative approach. Given that the analytical methods used in the toxicological studies were not available for most of the studies, the experts thus concluded that the genotoxicity potential of chlorpyrifos-methyl remains as unclear as that of chlorpyrifos, in particular with regard to DNA damage12. All agreed that these uncertainties should be considered in the risk assessment of chlorpyrifos-methyl. In the meantime and despite these considerations, the report specifies that “overall the data package did not show any concern and that all experts agreed that the genotoxicity data package in regulatory studies for chlorpyrifos‐methyl is complete and overall negative”.
No evidence for a carcinogenicity potential was found upon chlorpyrifos-methyl administration in rats or mice.
d) Reproductive/developmental toxicity
Chlorpyrifos-methyl did not affect the reproductive performance up to the highest dose tested in a two-generation reproductive toxicity study in rats. The report states that “no developmental adverse effects were observed in either rats or rabbits”, this while, in the rat study, the parental adverse effect related to AChE enzyme inhibition in red blood cells (RBC) and adrenal toxicity were critical with a NOAEL of 1 mg/kg bw/day and the NOAEL for AChE inhibition in RBC in pups was 3 mg/kg bw/day. Regarding maternal toxicity, in rats AChE inhibition in RBC and brain was the critical effect identified, while in rabbits no adverse effect was observed.
e) Immunotoxicity and endocrine disruption effects
On the immunotoxic potential of chlorpyrifos-methyl, a data gap was identified as no information has been provided.
For endocrine-mediated and endocrine disruptive effects, a multigenerational study conducted according to the most recent test guideline showed no evidence of producing signs of overt toxicity (AChE inhibition). On this basis it was concluded that mechanistic studies are not required to assess the endocrine disruption potential of chlorpyrifos-methyl and all experts agreed that chlorpyrifos-methyl is not an endocrine disruptor in humans.
f) Developmental neurotoxicity (DNT)
In a developmental toxicity study (DNT) study in rats, the only chloropyrifos-methyl related effects observed were statistically significant lower AChE activity values in the RBC and the brain with AChE at 10 and 50 mg/kg bw/day compared to the control group. All the experts agreed to set a maternal No Adverse Observed Effect Level (NOAEL) at 2 mg/kg bw per day.
Regarding offspring toxicity, pup growth, survival and clinical conditions were unaffected. No test-substance related effects were observed13 at any dietary concentration at any age.
While a postnatal decrease in the height of cerebral hemisphere was observed in males at the top dose and a statistically significant inhibition of AChE in red blood cells in males, the experts considered that actually no reliable statistical analysis could be performed since just three control samples in females were available and even if, by combining the data observed in males and females, no effect on cerebellum height was shown.
On this basis, the majority of the experts considered that the results from some studies contribute to the evidence of DNT effects in humans due to the exposure to chlorpyrifos and chlorpyrifos‐methyl and occurring at doses lower than that causing 20% inhibition of AChE. Therefore, this would represent a concern to be taken into consideration for the risk assessment. All the experts, but one, agreed that, a specific No Adverse Effect Level (NOAEL) for developmental neurotoxicity (DNT) could not be set and that the Lowest Observed Effect Level (LOAEL) of 0.3 mg/kg body weight per day derived from the data on chlorpyrifos could be conservatively applied to chlorpyrifos-methyl.
Indeed, the DNT study on chlorpyrifos-methyl which did not show relevant effects was inconclusive as it had some significant limitations related to the controls, making a reliable statistical analysis impossible.
While the experts initially conservatively applied the same approach as for chlorpyrifos, considering that chlorpyrifos-methyl would also meet the criteria for classification as toxic for reproduction category 1B (regarding developmental toxicity), given that no toxicological reference values could be set, the developmental neurotoxicity potential of chlorpyrifos‐methyl was not further discussed.
11 Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006. OJ L 353, 31.12.2008, p. 1–1355.
12 For gene mutation, the experts considered that the results from the three bacterial and the two mammalian gene mutations assays overall showed that chlorpyrifos-methyl does not induce gene mutations in vitro; For chromosome aberration, from the results of two different assays in vitro chlorpyrifos-methyl was considered positive in some assays but negative in others both in the absence and in the presence of metabolic activation; Regarding unscheduled DNA synthesis, one in vitro study was submitted and produced negative results; For in vivo studies in somatic cells (mouse bone marrow micronucleus test) and rat liver DNA repair test (UDS): the studies showed no negative effects; Only in one in vitro chromosome aberration study positive findings in CHO cells in the presence of S9.
13 on body weights, body weight gains, attainment of developmental landmarks, detailed clinical observations, motor activity, auditory startle, learning and memory, macroscopic examinations and measurements, neuropathology or brain morphometry
In humans, epidemiological evidence based on data available is showing associations between chlorpyrifos and chlorpyrifos-methyl exposure during neurodevelopment and adverse health effects in children: attention deficit/ hyperactivity disorders, decrease in intelligent quotient and working memory, etc. A common metabolite of both chlorpyrifos and chlorpyrifos-methyl contributed to the evidence of DNT effects in humans.
The developmental neurotoxicity (DNT) effects observed at the lowest dose tested with chlorpyrifos indicating a health concern would be conservatively applied to chlorpyrifos-methyl. Given the severity of the effects, all the experts, but one, agreed that no No Oberved Adverse Effect Level (NOAEL) for developmental neurotoxicity (DNT) could be set and agreed, based on the DNT study available, on a Lowest Observed Effect Level ( LOAEL) of 0.3 mg/kg bw/day until there is no evidence for the contrary.
The subject was re-discussed during the meeting of September 2019 and the experts indicated that, based on the available toxicological (animal in vivo and in vitro) data set and the epidemiological evidence showing an association between chlorpyrifos/chlorpyrifos-methyl exposure during development and neurodevelopmental outcomes, chlorpyrifos‐methyl may be expected to meet the criteria for classification in accordance with the criteria set out with Regulation (EC) No 1272/2008 as “Toxic for the reproduction” and ‘May damage the unborn child’. The experts suggested thus to classify chlorpyrifos-methyl as “Toxic for reproduction”14.
EFSA expresses in the report some reservations on this approach, as based on the current experience the criteria for classification would normally be based on the specific effects recorded in good quality data. However, the European Chemicals Agency (ECHA) will be responsible for the final decision.
14 REPRO 1B, H360D ‘May damage the unborn child’ in accordance with the criteria set out in Regulation (EC) No 1272/2008 As highlight before, in this regulatory context, a risk represent the probability of exposure to an agent and cmpares this probability to the maximal residual level considered as acceptable and set a standard by the authorities EFSA Modification of the existing maximum residue levels for chlorpyrifos‐methyl in kaki/Japanese persimmon and granate apple/pomegranate https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2017.4838
On the basis of a consumer risk assessment15 results and provided that the conclusions of a Maximum Residue Levels (MRL) review are taken into consideration, EFSA concludes that the long‐term and short‐term intake of residues of chlorpyrifos‐methyl and its residue resulting from the existing and the intended uses in kaki/Japanese persimmons and granate apples/pomegranates fruits is unlikely to present a risk to consumer health16.
The estimated long‐term dietary intake of chlorpyrifos‐methyl residues was indeed calculated to account for up to 61% of the acceptable daily intake (ADI) and the estimated long‐term dietary intake of the residues accounted for up to 6% of the ADI.
On basis of the data available, the evaluating Member State (Spain in this case) proposed in an evaluation report on the basis of the consumer risk assessment to raise the existing MRLs in kaki/Japanese persimmons and granate apples/pomegranates from the limit of quantification to 0.5 and 0.3 mg/kg, respectively.
EFSA also concludes that for the crops assessed in this application, metabolism of chlorpyrifos‐methyl in primary crops is sufficiently addressed and the residue definitions proposed by the MRL review are applicable.
This assessment was made on the basis of an application and a dossier submitted to the competent national authority by Dow Chemicals to modify the MRLs in the fruits kaki/Japanese persimmons and granate apples/ pomegranates for the active substance chlorpyrifos‐methyl and one of its residues.
15 As highlight before, in this regulatory context, a risk represent the probability of exposure to an agent and cmpares this probability to the maximal residual level considered as acceptable and set a standard by the authorities
16 EFSA Modification of the existing maximum residue levels for chlorpyrifos‐methyl in kaki/Japanese persimmon and granate apple/pomegranate https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2017.4838
Is there a significant health risk from spray drift aerial exposure to chlorpyrifos?
In the U.S., the Human Health Assessment Branch of the Department of Pesticide Regulation (California Environmental Protection Agency)17 performed a very detailed assessment evaluating the dietary, spray drift, and aggregate risks that accompany exposure to chlorpyrifos from aerial and ground-based applications in the U.S. This risk assessment addresses potential human effects arising from exposure to chlorpyrifos from food, drinking water, air and skin contact with residues (horizontal deposition and aerosols associated with spray drift), incidental soil ingestion, ingestion of residues by object-to-mouth, hand-to-mouth, as well as aggregate exposures from various combined scenarios focused on 4 sentinel subgroups of the general population: infants (> 1 year old), children 1-2 years old, children 6-12 years old, and females of childbearing age (13-49 years old).
For inhalation exposures resulting from aerial and ground-based chlorpyrifos short-term duration (1 – 1.5 hours) spray drift near an application site to children and females of childbearing age from dietary sources (food and drinking water) and dermal exposures, no risks were identified from exposures to children and women of childbearing age from dietary sources (food and drinking water) and dermal exposures resulting from spray drift.
The air component of the exposure contributed up to 95% of the total aggregate exposure risk. Consequently, exposure to aerosols in the air near chlorpyrifos application sites was the main driver of the risk estimates of cholinesterase inhibition, especially for children 1-2 year olds, and thus substantiated the evaluation of chlorpyrifos as a Toxic Air Contaminant.
Meanwhile, potential health risks related to chlorpyrifos exposure were identified as:
The evaluated exposure scenarios were based on standard operating procedures for lawns and turf post-application, and assumed exposure times near the application site of 1-1.5 hr. In addition, infants and children 1-2 yrs were assumed to receive additional exposure (incidental oral) from spray drift deposition through mouthing activities.
The critical toxicological points of departure (PoDs) used to characterize the risk from exposure to chlorpyrifos were human equivalent doses estimated by pharmacokinetic modelling, adopted from the 2014 US EPA Revised Human Risk Assessment for chlorpyrifos. Using the 10% AChE inhibition endpoint and exposures estimated from spray drift following aerial applications of chlorpyrifos, human health risks were identified from hand-to-mouth exposure to children, from inhalation exposure to children and women of childbearing age, and from various aggregate exposures.
Data available from the California Environmental Contaminant Biomonitoring Program (CECBP) gives also an indication of background environmental exposure to chlorpyrifos and/or chlorpyrifos-methyl in several study groups via the measurement of an urinary metabolite, 3,5,6-trichloro-2-pyridinol.
Risks were calculated as margin of exposure (MOE), a ratio of the NOEL to the human exposure level. A target MOE of 100 was applied which is generally considered protective for all exposure scenarios against the chlorpyrifos toxicity. The critical toxicity factors considered were related to AChE inhibition which is cumulative in nature in the presence of concurrent background exposures for populations in areas of high chlorpyrifos use.
The results of the current assessment found that the aggregate MOEs for a number of combined scenarios were below the target of 100. The air component contributed up to 95% to the aggregate risk. Default assumption of 10-fold were used due to database uncertainties in the pharmacokinetic model and to account for potentially more sensitive neurodevelopmental effects than AChE inhibition.
Important to note that recent in vivo animal studies provide evidence of neurotoxicity to developing organisms at chlorpyrifos doses below those causing cholinesterase inhibition (AchE). Effects noted include altered cognition, motor control, and behavior in rats and mice. The most important implication of the studies is that the threshold for chlorpyrifos-induced neurodevelopmental effects following exposure in early life may be 10-fold lower than the reported threshold of 1 mg/kg/day established for AChE inhibition in RBCs. The report recognizes that there is a potential for other effects occurring at chlorpyrifos concentrations lower than those that inhibit cholinesterase. There could be other modes of action and adverse outcome pathways leading to neurodevelopmental effects, including non-cholinergic systems, the endocannabinoid system, other signalling pathways, and oxidative stress.
17 Final Evaluation of Chlorpyrifos as a Toxic Air Contaminant. Human Health Assessment Branch of the Department of Pesticide Regulation (California Environmental Protection Agency) 2018 
| References: |
|---|
| Statement on the available outcomes of the human health assessment in the context of the pesticides peer review of the active substance chlorpyrifos-methyl – updated statement November 2019. European Food Safety Authority ()EFSA) https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2019.5908 |
| Modification of the existing maximum residue levels for chlorpyrifos‐methyl in kaki/Japanese persimmon and granate apple/pomegranate European Food Safety Authority (EFSA) https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2017.4838 |
| Final Evaluation of Chlorpyrifos as a Toxic Air Contaminant. Human Health Assessment Branch of the Department of Pesticide Regulation (California Environmental Protection Agency) 2018 |
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