Objectives
The last time you grabbed an apple, rinsed it off and took a bite, did you think about pesticide residues on it? What about the banana or green beans or the bread on your sandwich? If your food was not labeled "Organic" it came to you with pesticide(s) added. In this unit, I want my students to learn about potential health risks from common pesticide products in order to make informed decisions about their use and consider "green" alternatives. In the process of learning about health risks from pesticides, I will interject mathematics. Students will use government-published data to consider their level of exposure to pesticides from multiple sources: ingestion (food or water), inhalation (breathing), and dermal (skin contact). A critical math skill necessary for these calculations is converting units; therefore, the overriding mathematic theme of this unit will be Dimensional Analysis, sometimes referred to as the Factor-Label Method.
I teach at a comprehensive vocational-technical high school where students spend up to one-half of each day in their chosen career area and the remainder of their day in academic classes. The school is a "choice" public school and our students are held to the same academic standards as all public school students in the state. Our math classes are generally grouped heterogeneously and we find a wide range of abilities. Students choose our school for a variety of reasons. Some are focused on what they want to do when they finish high school and use the vo-tech school to get a head start; some have been moderately successful students and are looking for a route to success other than a four-year college, and some are avoiding their "feeder" school. All students ask the question, "Why do I need to learn this?"
By demonstrating that we come in contact with pesticides daily, in places we may not have considered, I think the students will quickly understand why they need to learn this. All of the data published in the government documents is given in metric units, but my students do not have a good sense for the magnitude of metric units. The mathematics in Dimensional Analysis will help them convert to units that are more familiar to them. The skills they learn in this unit will also prove useful in their science classes. While I think parts of this unit are general enough to be used for any high school, or even middle school, math class, I am planning to use it for students in Intermediate Algebra. At my school, it is actually the fourth math course our students take, and it fills in a lot of gaps left by the Integrated Math program that we use. The students are primarily juniors or seniors and many of them will continue their schooling at the local community college. They are beyond the Mathematics high-stakes state test, so interjecting this unit into the course will strengthen their number sense and measurement skills without compromising the curriculum. An added benefit is that our 11 th grade students take the Science state test, and Dimensional Analysis may help with the mathematical part of that.
Background Information
According to the Environmental Protection Agency (EPA) governmental website, the term pesticide covers many categories of substances depending on the type of pest they target, or the method in which they are produced (i.e. chemical pesticides versus biopesticides). Some examples of pesticides that students might recognize (but may not realize fit the category) are algicides (kills algae), defoliants (kills leaves on trees), disinfectants/sanitizers, fungicides (kills fungus), herbicides (kills plants), insecticides, repellents, and rodenticides (kills rodents). 1 Many common household products are pesticides, such as ant and roach killers, bug repellent, flea and tick collars for pets, lawn and garden products, kitchen, bath or laundry cleaning products, and swimming pool chemicals.
Before taking this seminar, "Urban Environmental Quality and Human Health" led by Professor John Wargo, I never thought of all the ways we can be exposed to pesticides. When food crops are sprayed the pesticide settles on the leaves and the "fruit" and also on the ground. That much I knew. But, the application of the pesticide is not really uniform. Depending on the machinery used, the level may be higher at the ends of rows while the tractor/machine is turning, or it may be higher in the center if the pesticide is being applied by airplane. Plus there's overlap of spraying areas, increasing the level applied to some fraction of plants, but sampling methods are not likely to find the differences. Then there's the pesticide that reaches the ground that may be absorbed into the plant, or may eventually seep into the groundwater. Or, the pesticide may get into surface streams or lakes from run-off after heavy rains. All of these exposures could reach humans in one step, but we can also be exposed to some persistent pesticides through the food chain.
We also talked about breathing contaminated air. For example, EPA has limits, called re-entry times (RET), for how quickly farm workers are allowed to return to an area after pesticides have been applied. Not that we know the exact process, but the pesticide would be in the air for some length of time before reaching the ground, plus there could be ongoing evaporation or volatility of some of the pesticide, and all of it can be affected by weather conditions. For a person in the area when a pesticide is applied, there could also be dermal exposure. The pesticide can touch the skin directly, or soak through clothing. And, other people can be exposed by touching someone else's contaminated clothing. 2
In addition to pesticides applied to farmland, or, on a smaller scale, our lawns and gardens, we use pesticides inside our homes. Again, I had never considered some of the risks we face at home. Our indoor air can be contaminated with the obvious pesticides for killing ants or roaches, or with sanitizers or additives to paint and carpet. If our windows are closed, the contaminants remain and can build up. Children are at a greater risk indoors for several reasons. First, they spend more time indoors than adults. Second, they are closer to the ground where pesticide residues will settle, and they have a natural habit of putting things, including fingers that have potentially touched the residue, in their mouths. Think about a child crawling on a floor after someone walked through after applying weed killer outside. (Fortunately, this kind of exposure can be eliminated easily by leaving work shoes outside!) 3
As I just described, we can be exposed to pesticides through multiple routes: ingestion (oral), inhalation, or dermal. The EPA sets limits for each of the possible exposure routes for each pesticide it registers. The figure below summarizes the possible routes and sources of pesticide exposure that I descrived. 4 In the figure, "Residential Pathway" refers to pesticides used for lawns, gardens, golf courses, schools, pets, and as common household items.
EPA tolerances (maximum allowable level) for pesticides are set based on a No Observable Adverse Effect Level (NOAEL, or sometimes NOEL). The testing process for determining NOAEL is done with animals, so the scientists must do some extrapolation. First they produce a dose response curve (some measure of response in the animal versus dose of pesticide) and find the benchmark dose that produces a response in 10% of the subjects, called BMD 1 0. The pesticide level (dose) at that point is known as the Point of Departure (PoD), and is used to extrapolate to the human risk. Since extrapolation is merely a prediction of behavior, the PoD is then divided by a safety factor of at least 100, and up to 1000 in an effort to provide a safety margin of exposure (MOE), even a margin great enough for children. 5 As I mentioned earlier, children's exposure to pesticides represents an even greater risk than adults' because of their behavior (crawling, hand-to-mouth). But, children's exposure is also greater because of their size and metabolism. For example, children consume more food, and more of a smaller variety of foods per pound of body weight than adults. In addition, children's respiration rates are higher than adults' accounting for their relative body size. 6
In 1996 the Food Quality Protection Act (FQPA) tightened some of the regulations for registering pesticides. The law requires a manufacturer to demonstrate General Safety Standards (tighter tolerances for residues on food). It shifted the burden of proof to the manufacturer to demonstrate the safety of registered pesticides. The new law also considered aggregate risk (multiple sources of one chemical) and cumulative risk (multiple chemicals with similar mechanisms), and required review of all pesticide tolerances to ensure they met the new standards by 2006, beginning with the pesticides posing the greatest risk. Probably the most important and far-reaching part of the Act was increasing protection for children. It is because of the FQPA that the safety factor I referred to earlier was increased by another factor of ten, becoming 1000. 7
In our seminar, "Urban Environmental Quality and Human Health," we discussed several reasons to be wary of tolerances and reported exposures for pesticides. One reason is how sampling is done to test for pesticide residues. For example, only small quantities of food might be tested and found to be pesticide-free while a small (but dangerous) quantity exists that has a high level of toxicity. Or, pieces of food are blended together to determine an average residue level. The average level may be low, but an individual food item could contain pesticide levels with MOE values below 100 for adults, or below 1000 (or an accepted lower value) for children. The percentage of these cases may be low, but even if 0.1% of the US population eats an apple a day, it is still 300,000 potentially poisonous apples! There is even more uncertainty because pesticides are typically mixtures of several chemicals, and, currently, there isn't any testing done to learn about interactive or cumulative effects of multiple chemicals; EPA tolerances are only for individual chemicals. Not only that, but the inert ingredients in a pesticide may also be harmful chemicals, but are not considered in pesticide residue data. 8
One common class of pesticides, called organophosphates, affects the nervous system of pests, and humans if the exposure level is high enough. While it may not be the best way to determine their toxicity, measuring the cholinesterase enzyme level in blood is the accepted method for measuring exposure to organophosphate pesticides. In the human body, stimulating signals for muscles are carried across synapses in the nervous system by acetylcholine. These stimulating signals are turned off by the enzyme acetylcholinesterase, and this reaction occurs very quickly and often. However, if a person is exposed to a high enough level of an organophosphate pesticide, there will not be enough of the cholinesterase in the synapses and acetylcholine can build up. That means the muscles will remain in a stimulated state, moving uncontrollably, twitching, possibly causing convulsions or paralyzed breathing, and even death. 9 Sometimes the symptoms of organophosphate poisoning are misdiagnosed because they appear similar to flu symptoms.
One of the most commonly used organophosphate pesticides, chlorpyrifos, is a broad-spectrum pesticide. Prior to 2002 it was widely used on many crops, by professional exterminators for indoor and outdoor applications in commercial buildings, schools, daycare centers, hotels, restaurants, hospitals, stores, warehouses, food manufacturing plants and vehicles (cars, trains, airplanes, etc.). It was also used by homeowners to spray for bugs around the house. It is effective at killing many species of bugs, including termites, mosquitoes, ants, ticks, fleas, grubs, and termites. 10 Late in 2001 it was banned for indoor uses and no longer sold to nonprofessional users primarily because of potential harm to young children. It was also banned for use on tomatoes and limited for use on grapes and apples because they are a large percentage of children's diet. 11
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