A new study published in the Journal of the American Heart Association, September 25, 2019, reports that the risk of cardiovascular disease (CVD) is increased by 42% in Japanese-American men exposed to high levels of pesticide. This is the first time this outcome has been explored in this group, and indicates that the role played by occupational pesticide exposure in the pathogenesis of CVD needs to be examined.
CVD is the leading killer of today, and is responsible for almost a third of all deaths. One cause of CVD appears to be exposure to pesticides which seems to have a negative impact on the cardiovascular system. The data comes from the Kuakini Honolulu Heart Program, which is an ongoing study on Japanese-American males. It tracks all causes of death and some disease outcomes in this group, which has been examined many times over the study period.
Farmers spraying pesticide in wheat field. Image Credit: Jinning Li / Shutterstock
Pesticides – the issue
Many studies have shown that using pesticides, especially without proper protective equipment, can increase the rate of CVD, including ischemic heart disease and diabetes mellitus. For instance, these have been shown to be associated with phenoxy herbicides and chlorophenol use. Other chemicals used in agriculture have also been linked to heart attacks, heart failure, strokes, irregular heart rhythms, and even sudden death.
Hawaii uses several types of pesticides, including organophosphates and organochlorines, insecticides, herbicides and fumigants. It is important to know that most of these agents were originally used to wage chemical warfare in the two World Wars, and it was not until the close of the Second World War that they began to be used in agriculture. Subsequently, many have been found to be persistent in ground and organic matter for decades, leading to a ban on their use.
Pesticides today are used by farmers and other agricultural workers, as well as for industries that involve manufacture of pesticides, aircraft mechanics, those who work in certain oil refineries, and also by forestry workers.
Older research showed a link between pesticide (and solvent or metal) exposure and overall mortality from diseases of the circulatory and respiratory systems or from cancer. Among these, the frist two showed an association with pesticide exposure. Especially, the death rate from CVD at 15 years from exposure was significantly associated with this risk factor.
The study – how it was done
The present study differs from prior research in exploring the link between pesticide exposure and not just CVD but also stroke and coronary heart disease (CHD) individually. It aims to find out if this is a risk factor for such diseases.
The study included about 8,000 men of Japanese-American men on Oahu, Hawaii, aged 45-68 years. Researchers collected data on their occupations, the incidence of new CVD, and the Occupational Safety and Health Administration score to evaluate the level of intensity at which each type of occupation was exposed. Some parameters of the OSHA scale include permissible exposure limits, which means the maximum amount of the chemical to which a person can be safely exposed, over a time-weighted average, that is, the average amount to which the person is exposed over a defined period. This is typically a workday (8 hours) or a work week (40 hours).
These scores were analyzed along with the years worked, the age during which this work was carried out, and the type of occupation. This data was used to classify the participants into four categories: low, medium, high and no exposure. Exposure categories were determined on the basis of whether any exposure occurred, for how long, at what level, and at what time.
Outcomes were determined based on the hospital and death records, with regular examinations and autopsy records, looking for CVD, CHD, and strokes, for a follow-up that lasted at most 34 years. Other risk factors that could have contributed to the outcome were identified and accounted for in the analysis, such as smoking, age, blood pressure, cholesterol levels, alcohol, exercise, weight and education.
Pesticide exposure vs CVD – Study outcomes
New CVD rates were 43% higher in patients who were older, had heavier bodies, higher cholesterol and glucose levels, and smokers, but less in those who drank. Pesticide exposure was increased in older patients, those who were more active physically, had lower triglyceride levels, drank less, and had a lower educational status. Increased physical activity was to be expected in this group comprising mostly manual workers. This factor could also have masked the actual incidence of CVD in this group.
When the CVD incidence was compared to the levels of pesticide exposure, the highest incidence over 10 years was associated with the highest exposure levels. The next highest incidence was in the no-exposure group.
However, there is no continued association in the longer 34-year follow-up. This may indicate that aging brings its own set of risk factors for CVD which masks the effect of pesticide exposure. There was no significant link between CHD or stroke and pesticide exposure, probably because there were too few cases.
Low and moderate exposure levels had a lower risk but the difference was insignificant. Some experts invoke the hormesis principle to account for this: low exposure to some toxins induces a protective effect and stimulates compensatory changes in the organism to maintain a healthy state. In this case, the low exposure to pesticides could induce protective enzymes to be produced at a higher level, which means that higher exposures at a later date can be more successfully dealt with. In the current study, the same effect is seen with alcohol consumption which in moderate amounts is known to reduce CVD.
How high pesticide exposure increases CVD risk
CVD in younger people exposed to high pesticide levels could be due to the effect of organophosphates on the neurotransmitter called acetylcholine, which acts in harmony with the enzyme acetylcholinesterase that breaks it down. Thus the required amounts of this chemical are present to modulate all neuromuscular junctions and hence regulates muscular activity in the skeletal, cardiac and visceral muscle.
Another mechanism is via the raised levels of fats in the blood which predisposes to arterial disease and heart attacks, stroke and angina pectoris. The fatty changes are due to pesticide-induced liver damage.
Genetic factors are also implicated, as the PON genes, for instance, code for enzymes that regulate cholesterol synthesis as well as neurotransmitter breakdown. The different variants of the gene that occur in different populations may give the individuals higher or lower susceptibility to chemicals like pesticides that affect these enzymes. The study also gives no insights into pesticide-related CVD risk in women or other ethnic groups due to its monotonic composition.
The study shows that occupational exposure to high levels of pesticide is an independent risk factor for CVD in men within ten years, and the effects may linger because of the long persistence of these chemicals in the body. According to researcher Beatriz L. Rodriguez, it “emphasizes the importance of using personal protective equipment during exposure to pesticides on the job and the importance of documenting occupational exposure to pesticides in medical records, as well as controlling standard heart disease risk factors.”
Zara K. Berg, Beatriz Rodriguez, James Davis, Alan R. Katz, Robert V. Cooney, and Kamal Masaki, Association Between Occupational Exposure to Pesticides and Cardiovascular Disease Incidence: The Kuakini Honolulu Heart Program, Journal of the American Heart Association. https://www.ahajournals.org/doi/full/10.1161/JAHA.119.012569