July 2000 Volume 6 Number 7 pp 729 - 731
Balancing risks on the backs of the poor
Amir Attaran2, Donald R. Roberts1, Chris F. Curtis3 & Wenceslaus L. Kilama4
1. Department of Preventive Medicine and Biometrics Uniformed
Services University of the Health Sciences Bethesda,
Maryland 20814, USA
2. Center for International Development Kennedy School
of Government Harvard University Cambridge Massachussetts
3. London School of Hygiene & Tropical Medicine
London WC1E 7HT, UK
4. Chairman, Malaria Foundation International; also Chairman-Coordinator,
African Malaria Vaccine Testing Network C26/27 Tanzania Commission for Science
and Technology Building, Ali Hassan Mwinyi Road, P.O. Box 33207
Dar Es Salaam, Tanzania
Malaria kills over one million people, mainly children, in the tropics
each year, and DDT remains one of the few affordable, effective tools against
the mosquitoes that transmit the disease. Attaran et al. explain that
the scientific literature on the need to withdraw DDT is unpersuasive, and
the benefits of DDT in saving lives from malaria are well worth the risks.
Few chemicals stir the feelings of the 'man on the street'
quite like DDT (dichlorodiphenyltrichloroethane). Since Rachel Carson's
Silent Spring, conservationists in rich, developed countries have waged
a decades-long campaign, no less persistent than DDT itself, to convince governments
and citizens that DDT is an irredeemable pollutant. They have been very successful:
Every industrial country, without exception, has ceased using DDT.
However, DDT remains one of the few affordable, effective tools against
the mosquitoes that transmit malaria, a plague that sickens at least 300 million
and kills over one million, mainly children, in economically underdeveloped
areas of the tropics each year. Such a toll is scarcely comprehensible. To
visualize it, imagine filling seven Boeing 747s with children, and then crashing
them, every day.
Until now, developed countries have grudgingly tolerated the use of DDT
against malaria in poor tropical countries; at least 23 countries do so1. However, this may now be ending. Led by the United Nations Environment
Programme, more than 110 countries are negotiating a treaty to "reduce
and/or eliminate...the emissions and discharges" of 12 persistent
organic pollutants, citing their "unreasonable and otherwise unmanageable
risks to human health and the environment."2 If it becomes
law, the treaty will likely end DDT manufacture, or at least make the supply
scarce and unaffordable to tropical countries.
This, in our view and that of nearly 400 colleagues who have signed an
open letter to the diplomats negotiating the treaty, is simply dangerous3. The scientific literature is unpersuasive of the need to withdraw
DDT; on the contrary, it is clear that doing so risks making malaria control
ineffective, unaffordable, or both.
DDT became emblematic of the toxics movement because of its effects on
the non-human environment. Ecological studies have demonstrated that bioaccumulated
DDT could cause thinning of eggshells and reproductive failure in birds of
prey. The fault for this lies in the massive agricultural use of DDT. Dusting
a single 100-hectare cotton field, for example, can require more than 1,100
kg of DDT over 4 weeks4.
In contrast, DDT spraying for malaria control is less intensive, less frequent
and far more contained. The current practice is to spray the interior surfaces
only of houses at risk, leaving a residue of DDT at a concentration of 2 g/m
2 on the walls, ceiling and eaves, once or twice a year. Half a kilogram
can treat a large house and protect all its inhabitants. Doubtless some fraction
of this escapes to the outdoors, but even assuming it all did, the environmental
effect is just 0.04% of the effect of spraying the cotton field. Guyana's
entire high-risk population for malaria can be protected with the DDT that
might otherwise be sprayed on 0.4 km2 of cotton in a season5. Compared with its agriculture uses, public health uses of DDT are
too trivial to merit banning with any urgency.
Environment aside, health considerations arise, and with them the dilemma
that one man's benefit is another man's risk. Environmentalists in rich, developed
countries gain nothing from DDT, and thus small risks felt at home loom larger
than health benefits for the poor tropics. More than 200 environmental groups,
including Greenpeace, Physicians for Social Responsibility and the World Wildlife
Fund, actively condemn DDT for being "a current source of significant
injury to...humans."6 But five decades of experience
with DDT shows that it is highly effective and safe when deployed in house
Reliance on DDT reached its zenith, and malaria, its nadir, with a campaign
to eradicate malaria from large parts of the world in the 1950s and 1960s.
The early results were impressive: in less than two decades, spraying of houses
with DDT reduced Sri Lanka's malaria burden from 2.8 million cases and 7,300
deaths to 17 cases and no deaths. India and South America achieved similarly
impressive reductions, and several countries fully eradicated malaria1. Even in sub-Saharan Africa, where mosquitoes are most difficult
to control, DDT spraying resulted in great reductions in malaria8.
Unfortunately, many of these successes were short-lived. American funds,
which underwrote the eradication campaign, soon lapsed, and overuse of DDT
in agriculture bred DDT-resistant mosquitoes. Back in malaria's grip, Sri
Lanka returned to a half a million cases by 1969.
But despite 'resistance' in itself, DDT still works to alleviate
mortality and morbidity. Resistance tests work by measuring whether mosquitoes
survive a normally toxic dose of DDT. The tests wholly overlook two non-toxic
actions of DDT: contact-mediated irritancy9, which drives mosquitoes
off sprayed walls and out of doors before they bite, and volatile repellency10, 11, which deters their entry in the first place. Both actions
disrupt humanmosquito contact and disease transmission.
Data from the Pan-American Health Organization show a strong inverse correlation
between malaria cases and rates of spraying houses (19591992) in South
America, even after DDT resistance became widespread in the 1960s
Fig. 1. Here, 'cumulative cases' represent the population-adjusted,
'running' total of cases that exceed or fall short of the average
annual number of cases from 1959 to 1979 (years in which World Health Organization
strategy emphasized house spraying12). Cumulative cases increase
considerably in later years, coincident with a sharp decrease in rates of
This inverse correlation is readily understandable because it is so biologically
plausible. For mosquitoes, DDT is a toxin, irritant and repellant all rolled
into one chemical. All three properties decrease the odds of being bitten
by mosquitoes, and toxicity particularly reduces the odds that parasite-bearing
mosquitoes will survive to infect others. Lowering these odds slows disease
propagation by second- or higher-order relationships and therefore is very
important13, 14. Indeed, renewing the spraying of houses with
DDT, as Ecuador did in the early 1990s, rapidly decreases case rates5.
This body of evidence is so indisputable that even environmental groups
such as Physicians for Social Responsibility concede that DDT is "highly
effective" in malaria control15. Campaigning for a DDT
ban given this benefit would seem politically difficult unless one alleged
even greater health risks associated with its use, which is precisely what
environmentalists do. Recent bulletins from Physicians for Social Responsibility
and the World Wildlife Fund cite animal studies indicating involvement of
DDT in neurological and immune deficits, and epidemiological studies linking
DDT to human cancers and endocrine-disrupting effects, such as reduced lactation15, 16.
In this kind of 'balance of risks' paradigm, the evidence must
be scrupulously weighed. Although the International Agency for Research on
Cancer rates DDT as a possible human carcinogen (along with, notably, several
pharmaceutical drugs), not one case-control study of DDT's human carcinogenicity
has been affirmatively replicated. Breast cancer furnishes the clearest example:
the first study to correlate DDT exposure with statistically elevated risk17 has now failed to be replicated at least 8 times18-25,
and of these later studies, some found exposure to significantly reduce risk24, 25. Much the same can be said of studies indicating involvement
of DDT in multiple myeloma, hepatic cancer and non-Hodgkin lymphoma26, 27.
That DDT interferes with maternal lactation is also questionable. The leading
study to correlate DDT metabolites in breast milk with unexplained, premature
weaning28 does not reach statistical significance unless the
data are first 'adjusted' for potential confounders, but the authors
did not disclose how their adjustment was done, and other labs have yet to
confirm the result28. Lactation's many social, nutritional and
cultural influences make inferences difficult, but even if DDT really abridges
lactation, the authors found a "lack of any detectable effect on children's
With such weak evidence of harm to human health, one must decide whether
to set policy as a precaution and ban DDT based on animal studies. Ordinarily,
this makes sense (given the alternative of experimenting on humans with toxins),
but not for the spraying of houses with DDT. Acting with precaution because
there are risks in animals, and thus denying people the known health benefits
of malaria control, is very unethical: house spraying exposes millions of
people to DDT, any of whose health can be studied, making extrapolations from
animal studies unnecessary. Proper case-control studies should be done before
policy is cast in treaty law.
Indeed, if precaution is relevant, it favors spraying houses with DDT,
because it is affordable or effective where other interventions may not be.
Cost data from India show that, even using DDT alone, the entire national
malaria-control budget is sufficient to protect only 65% of high-risk persons.
Switching to malathion, the next-cheapest alternative, reduces that coverage
to 21%, which leaves 71 million more persons unprotected29.
House spraying also has the advantage that it protects whole families, which
is sometimes overlooked in comparing it with insecticide-treated bed-nets,
which protect only one or two people at a time30. Simply put,
there are too few economic studies to determine with certainty whether bed-nets
are more or less cost-effective than DDT house spraying31. However,
the fact that spraying houses with DDT can lower the prevalence of malaria
parasitemia in highly endemic African communities to levels below that achieved
by bed-nets (less than 5%) indicates that it is careless to treat them interchangeably8.
Patience in all things
How then to reconcile DDT's 'Janus-faced' character? Its benefit
in alleviating the suffering of malaria, at a reasonable cost, outweighs any
reasonable speculation of its health risks. Living with this may not be easy;
changing it is harder still.
Above all, rich countries must allow, and even facilitate, poor tropical
countries to make choices about DDT freely, and with informed consent. African
countries in particular lack the resources to dispatch health experts to the
treaty negotiations, and although it provides financial assistance, the United
Nations Environment Programme has declined to assist with this, or even to
provide a translator when French- and English-speaking diplomats meet to discuss
DDT. The resulting lack of knowledge suffocates debate. At worst, threats
are used, as Belize learned when the US Agency for International Development
demanded that it stop using DDT.
Such arm-twisting is as lamentable as it is effective. Highly indebted
poor countries must of necessity rank poverty reduction over environmental
orthodoxy, and stimulating growth and foreign investment will require nearly
eliminating malaria from economically productive zones. This is essential
for development in sub-Saharan Africa, where malaria subtracts more than one
percentage point off the gross domestic product growth rate, for a compounded
loss (since 1965) now reaching up to $100 billion a year in foregone income32.
Seen in this way, the insistence to do without DDT is 'eco-colonialism'
that can impoverish no less than the imperial colonialism of the past did.
Sub-Saharan Africa, which never experienced much spraying of houses with DDT,
should consider starting this. South Africa, Swaziland and Madagascar, among
others, run successful DDT-spraying programs and prove it can be done1, 33.
At present, the United Nations Environment Programme mandate to "reduce
and/or eliminate" DDT probably cannot be accomplished safely, without
causing extra disease. As 'preachers of precaution', environmental
groups and rich country governments should start by committing at least $1
billion annually to roll back malaria in Africa. That is the sum requested
by African leaders at their first-ever Malaria Summit earlier this year34. Meeting this request is a small price to pay for respecting the
lives of the poor, and will bring us much closer to no longer needing DDT.
Denying it, on the other hand, is to again embrace indifference, and the pursuit
of environmental goals, on the backs of world's sickest and poorest.