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

July 5, 2021

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

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

 

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
http://www.who.int/ipcs/publications/ehc/ehc241.pdf

 

Authors:

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.”

WEEL OEL

Occupational Exposure Limits (OELs) are designed to safeguard the health of healthy workers during their careers. These limits are based on the assumption of repeated daily exposure throughout a working lifetime, typically averaged over an 8-hour workday. Their purpose is to prevent both immediate (acute) and long-term (chronic) health issues arising from workplace exposures. It’s important to note that OELs are not intended for the general public, which includes vulnerable groups like infants, the elderly, and those with pre-existing health conditions.

Workplace Environmental Exposure Levels (WEELs) are health-based guidelines for chemical hazards in the workplace. These values represent air concentrations believed to protect the majority of workers from negative health effects resulting from occupational chemical exposure.

The WEEL Process
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The development of new or revision of existing WEELs is typically assigned to voluntarily designated subcommittees. A subcommittee usually comprises 3 – 4 members from the WEEL Committee. New WEELs are developed using the OARS-WEEL administrative standard operating procedure (SOP), while existing WEELs are usually revised every 10 years, unless the availability of significant new data which may impact the existing WEEL value compels the committee to make a revision sooner. The OARS-WEEL SOP contains procedures and guidelines governing conflicts of interest, draft document preparation, literature searches, draft document review, balloting process, post-ballot WEEL documentation quality assurance scientific review, and publication.

Once a subcommittee has prepared a draft WEEL document, a review of the draft is scheduled for the next available Committee meeting. The WEEL Committee members are expected to have reviewed all such drafts prior to the meeting. If no major changes are necessary to a draft, the attending Committee membership may, by a simple majority, approve the WEEL for balloting. Alternatively, the Committee may direct the subcommittee to revise the WEEL and present it for further discussion at a future meeting. If a ballot is not approved by a two-thirds majority of non-abstaining Committee members, it is discussed at the next Committee meeting to determine the appropriate course of action. Once the WEEL is approved by a two-thirds majority of non-abstaining Committee members, copies of ballot comments are forwarded to the designated subcommittee and all substantive comments must be addressed in the final draft. If resolution of a substantive comment results in a change to the WEEL value or a change in the basis for the value, the draft must be re-balloted.

Once all comments have been addressed on a successfully balloted draft, document formatting and editorial review are performed by TERA, before the draft WEEL document is made available for public comment (usually for a period of 30 days but may be extended if the need arises). After the public comment period has elapsed, comments are addressed by the subcommittee responsible for that specific draft, after which the WEEL documentation is submitted to Toxicology and Industrial Health (TIH), a peer-reviewed medical journal that covers research in the fields of occupational health and toxicology, for publication. A thorough review of the galley proof by the scientific content quality coordinator at TERA, and proofreaders and editors at TIH is the penultimate step before eventual publication of the WEEL documentation.

The WEEL Committee

The OARS-WEEL Committee is composed of volunteer experts specializing in the scientific determination of occupational exposure levels. This committee actively seeks a balanced representation of professionals from toxicology and industrial hygiene, drawing upon a diverse range of experience from industry, government, academia, and consulting. Importantly, each member contributes to the Committee based on their individual expertise and not as an official representative of their respective employer, organization, or agency.