A true story of chemical risk and benefits: Rats, Cats And Parachutes

July 5, 2021


Our story is about a pesticide. People are often worried about pesticides because they are toxic chemicals. Pesticides are toxic so that they can kill things - like bugs. DDT is a bug spray. DDT was outlawed in many countries because once you spray it, DDT breaks down only very slowly in the environment. Since it stays around for a long time, it moves up the food chain and when birds eat bugs sprayed with DDT the chemical causes their egg shells to become thin. The egg shells become so thin that the mothers break the eggs when they sit on them, and the baby birds die. In fact, DDT almost wiped out falcons, hawks, condors and pelicans. After DDT was outlawed, the numbers of birds increased. So at first glance, banning DDT seems like a smart move.

Malaria is a disease with high fever, shaking, chills, vomiting and sweating that you can get from a mosquito bite. Did you know that there are 300,000,000 to 500,000,000 cases of malaria every year? And that nearly 3,000,000 people die every year from malaria? Did you know that most of those who die are children less than 5 years old? Mosquitoes are responsible for infecting people with malaria and people with malaria die when the malaria parasite gets into their liver and brain. If you want to stop malaria you need to kill the mosquitoes that carry the parasite. The chemical DDT is great if you want to kill mosquitoes since it is toxic to them and usually it does not hurt people.

The places with the most malaria are places with lots of mosquitoes. Borneo (an island we now call Indonesia) is a place with lots and lots of mosquitoes and lots and lots of malaria. Between 1952 and 1955, the World Health Organization began spraying DDT in Borneo to kill mosquitoes to control malaria. The chemical was sprayed on house walls and under beds so it would kill the mosquitoes before the mosquitoes could bite the people who lived there. Sure enough, the DDT killed lots of mosquitoes and since there were fewer mosquitoes, only a few people got malaria. But cockroaches aren't bothered much by DDT and the local caterpillars learned to avoid the DDT. As it turns out, jungle wasps that normally laid their eggs in the caterpillars died from DDT. The cockroaches were fine and without the wasps, the numbers of caterpillars increased and increased. Then the caterpillars started eating the thatched roofs of the houses. At the same time, the cockroaches and caterpillars that had been sprayed with DDT were eaten by geckoes (yes that same long-tailed lizard that appears in the car insurance commercials on TV). This is because geckoes like to dine on caterpillars along with a side of cockroach.

Because DDT accumulates up the food chain, the geckoes who ate the wasps, roaches and caterpillars that had DDT in their bodies also accumulated lots of DDT. The geckoes got sick. The sick geckoes slowed down. The island cats walked all around inside the houses and they usually entertained themselves by hunting and eating geckoes. As the cats walked through the houses the cats got DDT on their paws then they licked their paws and in so doing, they ate some DDT. When the geckoes became slow, the cats were very pleased and they ate as many geckoes as they could. Without enough geckoes and now that the wasps were gone, there were more and more caterpillars and they began to eat more and more of the thatch that was used to make the roofs on the houses. Then the roofs of the houses fell in.

But things only got worse.

The cats eating the geckoes only lasted for a little while, because the cats were poisoned by the DDT that was on their paws and inside the geckoe bodies.  The cats died.  When the cats were gone, the rats on Borneo declared a holiday and they had lots and lots of rat babies.  When all those rat babies grew up, this increased the numbers of fleas that lived on the rats.  Those nasty fleas can carry deadly bubonic plague (The Black Death) and plague can be transmitted from the fleas to the people who lived in the houses with the broken roofs.

So, now what?

What to do about all these rats with fleas and this new risk of plague? Well, the villagers decided they needed more cats to get rid of the rats and they decided to ask for help. They went to see the British Royal Air Force. So together with the people who lived in the houses with the broken roofs, the Royal Air Force helped the village to find some cats from the British Commonwealth of Malaysia and the Air Force parachuted cat reinforcements onto the island and the fresh cats took up the work of killing the rats..

But all of these efforts did not get rid of all of the mosquitoes or the malaria. Today the World Health Organization is still responsible for stopping malaria and the World Health Organization is stuck with a choice. Should we keep using DDT to kill mosquitoes? Or should we stop using DDT? On the one hand we have the real risk of death from malaria. On the other hand we have the toxicity of DDT to birds, to fish and to other animals. To make matters even more difficult scientists found out that when DDT was fed to laboratory mice it caused liver cancer and some studies suggested DDT might do the same thing in people. To decide what to do with DDT, the World Health Organization needed to know how dangerous DDT might or might not be: which is worse, to spray DDT to kill mosquitoes and take a chance with cancer or not to spray and let the mosquitoes do as they please? The World Health Organization tried other bug sprays, but these were even more toxic and they were more expensive than DDT (remember we are talking about many countries and many millions of people at real risk of death from malaria).


To help the World Health Organization make a decision about DDT, they looked at all the laboratory studies on DDT and then they used a computer to calculate the projected risk of cancer in people who lived in the houses that were sprayed with DDT. This projected risk is not the same as the real risk since this is an estimate based on what scientists measured in mice. Using their best estimate, the World Health Organization found that not more than 3 in 100,000 people would be at risk for cancer after spraying DDT.

How would you decide this question? How would you balance the health risks of DDT against its benefit to reduce malaria? What did the World Health Organization do? To this very day, the World Health Organization sponsors DDT spray inside homes in places like the jungles of Indonesia where malaria is common. This is because of the high numbers of people who die every year from malaria compared to the estimated (theoretical) numbers of people who might get cancer. So you can see the concept of "safety" is relative.


Fact Check and Resources  
Harrison, T. 1959. World Within: A Borneo Story.  London: Cresset Press.
O’Shaughnessy, P.T. 2008. Parachuting cats and crushed eggs. The controversy over the use of DDT.  American Journal of Public Health 98(11): 1940-1948.  

Royal Air Force. 1960. Operations Record Book. Flight of 48th Squadron. March 13. Changi, Singapore.  Compiled by Fg.Off.Humphrey

World Health Organization. 2006.  WHO gives indoor use of DDT a clean bill of health for controlling malaria. 
http:// www.who.int/mediacentre/news/releases/2006/pr50/en/

World Health Organization. 2011. Strengthening malaria control while reducing reliance on DDT.  http://www.who.int/ipcs/capacity_building/ddt_statement/en/index.html

World Health Organization. 2011. DDT in indoor residual spraying: human health aspects.  Environmental Health Criteria 241. 391 pp. Geneva: WHO



Calvin Willhite, Ph.D.
California Environmental Protection Agency (retired)

Michael L. Dourson, PhD., DABT, FATS, FSRA
Toxicology Excellence for Risk Assessment (TERA)

If you think parachuting cats is a fairy tale, the official Operations Record Book from March 13, 1960 taken down by the crew of a British Royal Air Force (RAF) Beverly transport plane from Singapore describes that it “carried out a unique drop to Bario in the Kelabit Highlands in Sarawak that included over 20 cats to wage war on rats”.  The Dayak people of that village thanked the RAF and thanked “all of the cat donors and cat basket makers” and added that “all of the cats are safe.”



June 4, 2021

Hey toxicologists, is organic food safer to eat?

By Michael L. Dourson, PhD., DABT, FATS, FSRA

Pick just about any newspaper or journal and during the course of a year, one or more articles will be devoted to the benefits (or not) of organic foods and the downsides (or not) of conventionally grown food with pesticides and herbicides. These articles are often confusing. 


So how does one sort them out?


Food, whether organically or conventionally grown, is a combination of chemicals, many of which our bodies need in order to function well. Organic food comes from plants grown without added antibiotics or growth hormones, pesticides, herbicides or genetic modification , whereas conventionally grown food may use one or more of these products.


However, not all chemicals in food are useful to our body, and some of them are harmful at a certain level, like too much aflatoxin---a natural fungal product---in peanut butter. And did you know the plants we grow for food naturally produce pesticides and herbicides to protect themselves from insects and weeds? Any gardener who has tried to grow tomatoes near a walnut tree can tell you this is true---the walnut tree’s roots produce a herbicide that is poisonous to tomato plants. The use of pesticides and herbicides, whether human-made or natural, often results in small levels of these chemicals in our food.


Genetic modification (GM) of a food crop, whether done in the lab or through traditional crossbreeding, is often one way to get the crop to develop a new pesticide or herbicide, or to increase the level of an already existing natural one. Such modification may also give the crop a way to resist damage by a human-made herbicide. So corn can be genetically modified in the lab to make a protein to protect it against insect damage and at the same time to resist damage by human-made herbicides used to kill weeds. The use of GM corn with both of these traits is popular because it not only increases yields, but also reduces plowing, soil erosion, and use of conventional pesticides and herbicides. Of course, corn has been genetically modified through traditional crossbreeding for years to increase yields and resist pests.


So now to the question!  Is organic food safer to eat?  Or perhaps, are the small levels of pesticides, herbicides and genetic modifications in our food, whether human-made or natural, harmful? 


Many organizations work on a daily basis to answer this latter question. In fact, tens of millions of dollars are spent by competing industries on appropriate experimental animal and exposure studies. These studies then are reviewed by toxicologists to establish safe levels. These levels are then compared with the anticipated exposures when the pesticide, herbicide or genetic modification is used according to its label. If the anticipated exposures are below the safe levels, then the uses are permitted and the small levels of these chemicals in our food are not harmful. Although such testing is generally not done on naturally occurring plant chemicals, human experience would suggest that exposures to many of these naturally occurring pesticides, herbicides or genetic modifications are also below safe levels, and thus, are also not harmful.


When comparing organic versus conventional methods of growing food, other issues may be important. For example, organically grown foods may better maintain a sustainable farm practice, may reduce unanticipated environmental impacts of a human-made pesticides, herbicides or genetically modified plants, and would reduce accidental and often demonstrated harm to workers from over exposure to these chemicals. However, organic farming generally needs extra plowing so soil erosion is increased when compared with conventional farming. And while eating organic foods has been demonstrated to lower consumer exposure to some human-made pesticides and herbicides, insect damage from unprotected plants can cause natural pesticides and herbicides to increase. Additionally, there is no strong evidence that organic foods are significantly more nutritious than conventional foods.


A Google query entitled “organic and conventionally grown foods, including GM foods” will yield many websites, many of which were judged by the scientific staff of Toxicology Excellence for Risk Assessment (TERA) as too biased, not credible, or non-related. However, some sources appear to of unbiased, credible and related. For example, nutritional content of food grown by either method does not appear to differ. Other sources can be accessed to answer numerous other questions.


Forman J. and J. Silverstein.  2012.  Organic Foods: health and environmental advantages and disadvantages.  Pediatrics: 2012-2579.  October 22.

Jason J. Hlywka , Gerald R. Stephenson , Mark K. Sears , Rickey Y. Yada.  1994. 
Effects of insect damage on glycoalkaloid content in potatoes (Solanum tuberosum).  J. Agric. Food Chem., 42 (11), pp 2545–2550.



May 4, 2021

Too much of a good thing?

By Michael L. Dourson, PhD., DABT, FATS, FSRA


We have all likely heard someone say something is “too much of a good thing.” This applies to foods, like ice cream, drinks, like wine, and many kinds of activities, like watching movies.  However, this also applies to chemicals to which we are exposed every day, or about which we often read in our daily news. 

In fact, all chemicals are toxic at some level – some can cause harm at very small concentrations, while others need a large amount before there is a danger to our health. For example, ingesting large amounts of dihydrogen monoxide can cause low blood sodium concentrations leading to nausea, fatigue, confusion and seizures, and even death, but few people would want to ban di-hydrogen (H2) mono-oxide (O)---also known as “water”--- from public sale and or other uses, since water is safe and necessary when we drink a normal amount.

For many chemicals, however, it’s difficult to know what level causes or does not cause harm. For example, most people know arsenic as a poison, but may not know that many foods contain small amounts of arsenic, since it is a part of our environment.  How much arsenic can people eat and not be harmed? 


You might remember the scare a while back about too much arsenic in apple juice.  The U.S. Food and Drug Administration analyzed a large number of apple juice samples for arsenic and compared them with their estimate of its safe dose, the level at which no harm was expected.  The FDA concluded that the very low levels of naturally occurring arsenic in apply juice were not a public health risk and the juice was safe for consumption.  Eventually FDA developed guidance for manufacturers.

Because of this difficulty, toxicologists have been trained to make a determination of levels where a chemical causes harm and where it does not, generally in experimental animals.  These determinations are then evaluated by scientists who specialize in risk assessment to make a judgment about the safe dose or safe level of the chemical to humans.   These risk scientists work in governments, industries, consultancies, and, to a less extent, universities and non-governmental organizations (NGOs).


These safe levels go by different names, a common one in the US is the reference dose (RfD).  But despite different names and slightly different approaches, risk scientists all follow three basic steps:

  1. Toxicity data are reviewed from laboratory studies generally on animals to confirm the levels of exposure that show harm and those which do not.  Sometimes important information is found from unintended human exposures.
  2. Uncertainties in the data and analyses are then considered, particularly when using animal studies, and a judgment is made of a chemical exposure that would be safe, even for sensitive groups of people (like children or the elderly).
  3. New research may be requested of toxicologists to resolve uncertainties or unclear data or analysis.


If you want to learn more about safe levels of chemicals, or how they are developed, many agencies provide a wealth of information.  For example:

  • U.S. National Library of Medicine, has useful toxicity information about chemicals on its websites (see http://toxnet.nlm.nih.gov);
  • U.S. Environmental Protection Agency also has a wealth of information on many of its websites (http://epa.gov; 
  • Society of Toxicology has its annual meeting in March (see https://www.toxicology.org/index.asp).
  • Toxicology Excellence for Risk Assessment assesses the risk to chemicals and shares this information (see http://www.tera.org); 
  • Toxicology Education Foundation has produced an excellent 15-minute video called "Is It Safe?" (http://toxedfoundation.org/is-it-safe-video-english-with-japanese-subtitles/) that provides information for the general public about chemicals and risk so that you can make good decisions associated with everyday products (see also other topics at: http://toxedfoundation.org).
  • American Council on Science and Health at https://www.acsh.org has essays on numerous topics many of them associated with chemicals and their safety.


These are only six examples of many groups that work on behalf of the public to keep us safe.



April, 2021

Who is a toxicologist and how is s/he different than my medical doctor?

Michael Dourson, PhD, DABT, FATS, FSRA 
Bernard Gadagbui, MS, PhD, DABT, ERT

Toxicology, the study of poisons, is often thought of as a new discipline. It’s not. It has been around as long as people have been trying out different types of food, and using the occasional poisonous plant, or animal, to dispatch a rival. Toxicology today is more disciplined, with scientists and medical doctors studying various ways the plethora of chemicals to which we are daily exposed to can either be of use, not of use, or downright dangerous. However, even sometimes very dangerous chemicals can otherwise be useful (think botox here, https://www.youtube.com/watch?time_continue=7&v=fFvYUdljVK0&feature=emb_logo). If you want to get a sense of toxicology that is readily understandable, here are 3 sets of facts that can be used around the proverbial office water cooler, or at your next party, should the conversation need a new direction:


  • Life is chemistry. We are exposed to tens of thousands of chemicals every day.  A cup of coffee has about a 1000 chemicals. Most garden vegetables have just as many, including many natural pesticides like solanine found in potatoes at high concentrations in all green parts (but even in the potato we eat at a safe level).  We could go on and on and…
  • All chemicals are toxic at some level. Yes, even water. Drink too much and it will kill you. Drink a little bit less and it will disrupt your endocrine system, specifically the hormones associated with your kidneys and adrenal glands.
  • All chemicals have a safe dose if they do not cause cancer or virtually safe dose if they cause cancer. Yes, they do!  It is the dose that makes any chemical a poison.  Even a very toxic chemical like arsenic, which has dispatched more than one famous person (Napoleon perhaps?), has a level below which folks do not worry about. Good thing too since arsenic is a naturally occurring element that can be found in nearly all water and food we eat daily if one looks closely enough.


So, what do toxicologists do every day?  Well, like many occupations, toxicology has a number of sub-disciplines.  The one we deal with on a daily basis is human health risk assessment and the related area of regulations.  This area is likely to be one in which you are most familiar, since fear of chemicals is prevalent in today’s social media and chemicals are often written up in mainstream news outlets in negative terms.  Most of these stories are short on the science and long on the scare, so beware and check the backgrounds of the author and those s/he quotes before you start believing anything.


Toxicologists adept at risk assessment and regulation can be found in government, industry, consulting and, to a much lesser extent, academic and NGO organizations.  Sections of scientific organizations are even devoted to risk assessment and regulation.  On a daily basis, these toxicologists review studies done on particular chemicals and mixtures to determine safe levels of exposure.


Other areas of toxicology will likely not surprise you.  Some toxicologists conduct chemical experiments on animals or cell cultures to determine the level at which effects occur.  Some study the structure of a chemical and compare it to chemicals that have similar structures for insights.  Some extract natural chemicals from plants and animals looking to make new drugs or other useful chemicals.  And all of this and other related work necessitate advanced training in biology, chemistry, physiology and pathology and other disciplines, often ending up with conferral of a masters or doctorate in toxicology.


So, where does your medical doctor fit in?  Well, some of the most famous toxicologists are also medical doctors, and so the degree, either an MD or PhD is really not as important in determining whether someone is a toxicologist as the underlying work (https://en.wikipedia.org/wiki/John_Doull_(toxicologist).  However, a useful distinction is that a medical doctor is often focused on your health and will get you into a hospital if this is needed.  A doctor of toxicology is concerned with preventing ill health from chemical exposure---we try to keep you out of the hospital.  Another useful way to look at toxicology is that it is preventive medicine.  We are trying to lessen the workload of our medical doctor colleagues by finding ways for you to avoid disease.


So, next time you read a social media post or newspaper article and have a question, send us a note, ask another toxicologist for some help, or go to one of these websites to check out relevant information:


Email dourson@tera.org; gadagbui@tera.org
Phone 513.542.7475 Ext: 105; 513.542.7475 Ext: 104
Mobile Phone 513.543.2892; 513.313.3160 

Questions to consider are: How many of the sources are from scientists?  How many of these scientists are toxicologists?  How many of these toxicologists, if any, are board-certified? 

The Society of Toxicology’s specialty sections for Regulatory and Safety Evaluation and Risk Assessment are two prime examples.