From the Guidelines

Chlorpyrifos can cause recurrent symptoms due to prolonged exposure, not recirculation.

The evidence suggests that chlorpyrifos exposure, even at low levels, can have neurodevelopmental effects on children, including cognitive, behavioral, and social development issues 1.
Prolonged exposure to chlorpyrifos has been linked to various health problems, including attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and reduced IQ 1.
The US Environmental Protection Agency (EPA) has identified numerous scenarios that could result in unsafe exposures for agricultural workers and bystanders, leading to a proposal to revoke all standards that permit residues of chlorpyrifos on food 1.
The evidence does not support the idea that recirculation of chlorpyrifos is a primary cause of recurrent symptoms, but rather that prolonged exposure to the pesticide is the main concern 1.

Chlorpyrifos exposure can occur through various routes, including ingestion, inhalation, and skin contact 1.
The health effects of chlorpyrifos exposure can be severe and long-lasting, with some studies showing that exposure during fetal development can lead to persistent cognitive and behavioral problems in children 1.
The use of chlorpyrifos and other organophosphate pesticides has been widely criticized due to their potential health risks, and many organizations are calling for a ban on their use 1.

From the Research

The available evidence suggests that chlorpyrifos exposure is not typically associated with long-term effects, unless the poisoning is acute and severe 2, 3.
Studies have shown that chlorpyrifos toxicity does not occur in the absence of significant cholinesterase inhibition, and if exposures are less than those that cause significant cholinesterase depression, then no signs or symptoms related to chlorpyrifos exposure occur 4.
However, some research suggests that chronic exposure to chlorpyrifos may lead to DNA damage, epigenetic alterations, and neoplastic transformation of liver cells, which could potentially cause recurrent symptoms 5.
Additionally, chlorpyrifos has been linked to various adverse health effects, including neurological disorders, liver and kidney dysfunctions, and cancer, particularly in cases of occupational or environmental exposure 6.

The mechanism of action of chlorpyrifos involves blocking the active sites of the enzyme acetylcholinesterase (AChE), producing adverse nervous system effects 6.
Chlorpyrifos has active metabolites, 3,5,6-trichloro-2-pyridinol (TCP), and chlorpyrifos-oxon (CPO), which are more persistent and equally toxic, producing serious health complications 6.

The weight of empirical evidence indicates that the risk of adults or children experiencing an adverse health effect from exposure to chlorpyrifos through both nondietary and dietary sources is negligible, unless the products are seriously misused 4.
However, some studies suggest that further investigation is needed to fully understand the chronic health implications of chlorpyrifos exposure, particularly in cases of occupational or environmental exposure 3, 5.