Range, Magnitude, and Duration of Effects of
Natural and Human-Caused Disasters:
A Review of the Empirical Literature
October 4, 2001
National Center For Post Traumatic Stress Disorder (PTSD)
This Study On The NCPTSD>
Prepared by: Fran H. Norris, Georgia State University, with the
assistance of: Christopher M. Byrne and Eolia Diaz, Georgia State
University, and Krzysztof Kaniasty, Indiana University of Pennsylvania
A total of 177 articles that described results for 130 distinct samples
composed of over 50,000 individuals who experienced 80 different disasters
were coded as to:
disaster type (62% natural disasters, 29% technological disasters, and 9%
disaster location (60% USA, 25% other developed country, 15% developing
sample type (73% adult survivors, 16% youth, 11% rescue/recovery workers)
several methodological variables
After a preliminary review of the studies, each sample was coded as to
the presence of 6 sets of outcomes and rated as to its overall severity of
Range of Outcomes
Specific psychological problems were identified in 74% of the
samples. Posttraumatic stress or PTSD was found in 65% of the samples,
depression or major depression disorder was found in 37% of the samples,
and anxiety or generalized anxiety disorder was found in 19% of the
samples. Panic disorder and specific phobias were rare.
Non-specific distress, assessed by means of global indices of
psychological and psychosomatic symptoms, was identified in 39% of the
Health problems and concerns, such as self-reported somatic
complaints, verified medical conditions, increased taking of sick leave,
elevations in physiological indicators of stress, declines in immune
functioning, sleep disruption, increased use of substances, and (if
previously disabled) relapse and illness burden, were identified in 25% of
Chronic problems in living, identified in 10% of the samples, were
assessed rarely but generally found where they were assessed. Such
problems included troubled interpersonal relationships, social disruption,
family strains and conflicts, excess obligations to provide support,
occupational stress, financial stress, environmental worry, and ecological
Psychosocial resource losses were also assessed less frequently
than the first 3 sets but nonetheless found in 10% of the samples.
Declines in perceived support, social embeddedness, coping self-efficacy,
and optimism were at least occasionally observed.
Problems specific to youth included various behavioral problems and
separation anxiety among children, and deviance and delinquency among
Magnitude of Effects
To provide a rough estimate of the overall impact of the events
studied, each sample's results were classified on a 4-point scale of
9% showed minimal impairment, meaning that the majority of the sample
experienced only transient stress reactions;
52% showed moderate impairment, wherein prolonged but subclinical distress
was the predominant result;
23% showed severe impairment, meaning that 25% to 49% of the sample
suffered from criterion-level psychopathology; and
16% showed very severe impairment, meaning that 50% or more of the sample
suffered from criterion-level psychopathology.
Variables Which Predicted the Sample's
Overall Severity of Impairment
School-aged youth were most likely, and rescue/recovery workers
least likely, to show severe impairment: 62% of the school-aged samples
experienced severe impairment, compared to 39% of the adult survivor
samples and 7% of the rescue/recovery samples.
Developing countries were at greatest risk when location of the
disaster was considered. Severe effects were observed in 27% of the U.S.
samples, 46% of the samples from other developed countries, and 79% of the
samples from developing countries.
Mass violence was, by far, the most disturbing type of disaster. Of
the samples that experienced mass violence, 67% were severely impaired,
compared to 34% of the samples who experienced technological disasters,
and 42% of the samples who experienced natural disasters.
Disaster type and disaster location interacted to predict the
sample's impairment. Almost all samples from developing countries
experienced natural disasters, many of which were catastrophic in scope,
involving high death tolls. Natural disasters in developing countries
yielded a higher mean severity rating than did either natural or
technological disasters elsewhere. However, within the developed
countries, technological disasters had a significantly higher aggregate
severity rating than did natural disasters. Thus, for the narrower purpose
of understanding the typical impact of disasters in the United States, it
is reasonable to expect that technological disasters, on average, will be
more psychologically stressful than natural disasters. Technological
disasters, however, were less disturbing than disasters of mass violence
in both the United States and other developed countries.
Together, these 3 variables (disaster type, location, sample type)
explained 30% of the variance in the sample's severity of impairment. The
multiple correlation was .54.
Disasters in the United States
It was possible to identify several well-known events that were
illustrative of disasters that had atypically weak, typical, or atypically
strong effects on psychological outcomes. Common denominators among
events and samples are outlined below.
Atypically weak disasters were associated mostly with
minimal impairment in the samples studied. These were exemplified by the
1989 Loma Prieta earthquake, the 1994 earthquake in Northridge,
California, and the 1982 flood/dioxin contamination in the St. Louis
Epidemiologic Catchment Area. Most samples in this group were not very
seriously exposed or experienced little social disruption or had access to
substantial personal and community resources.
Typical disasters were associated with moderate impairment in the
samples studied. These were exemplified by the 1981 flood in Kentucky,
Hurricane Hugo in 1989 in the Carolinas, and the 1979 nuclear accident at
Three Mile Island. The diversity of events in this category point to a
variety of processes that intersect to produce or protect against
prolonged stress and distress. The effects of highly destructive events,
such as Hugo, may be reduced by strong interpersonal and community
supports, whereas the effects of less destructive events, such as the KY
floods in Appalachia, may be heightened by a low-resource context. Even in
the absence of trauma and actual property loss, the effects of
technological accidents may be comparable in magnitude because of victims'
residual uncertainties, health concerns, and loss of trust.
Atypically strong disasters were associated mostly with severe or
very severe impairment. These were exemplified by Hurricane Andrew in 1992
in south Florida, the 1972 dam collapse in Buffalo Creek, West Virginia,
the 1989 Exxon Valdez oil spill off the coast of Alaska, and the 1995
bombing of the Murrah Federal Building in Oklahoma City. These events
caused massive destruction or threat to life and/or prolonged social and
financial disruption and resource loss.
Duration of Effects
Twenty-seven panel studies (studies in which the same individuals are
interviewed on multiple occasions) provided data on the course of
postdisaster distress. Three primary trends were observed:
First, the general rule, observed in the vast majority of studies, was for
samples to improve as time passed. These effects were not always simply
linear, as some outcomes sometimes improved for a while, then stabilized
or worsened for awhile, then improved again.
Second, levels of symptoms in the early phases of disaster recovery were
good predictors of symptoms in later phases. Delayed onsets of
psychological disorders were rare.
Third, symptoms usually peaked in the first year and were less prevalent
thereafter, leaving only a minority of communities and only a minority of
individuals within those communities substantially impaired.
Summary and Conclusions
A substantial amount of research pertinent to understanding the range,
magnitude, and duration of the effects of disasters has been published
over the past 20 years. A variety of events were studied in a variety of
ways, the samples were impressively diverse, and individuals' experiences
ranged from little more than inconvenience to severe trauma and loss.
Accordingly, it is not surprising that results varied, with some samples
showing only minimal and transient stress reactions and others showing
prevalent and persistent psychopathology. Several conclusions can be drawn
on the basis of the literature reviewed for PART I:
The range and distribution of outcomes suggests that a quality
assessment of victims' mental health should include, at minimum:
a retrospective diagnostic assessment of PTSD, preferably one that anchors
the symptoms to the disaster
a brief measure of current nonspecific distress
an inventory of the acute and chronic stressors and resources losses
associated with the event.
Allowing 20 questions for (1), 10 questions for (2), and 20 questions
for (3), a 50-item screening tool could be developed for use in the field.
Such a measure could be completed by most adults in 20 minutes or less.
The relative risk of sample types, in which youth were at
greatest risk and rescue/recovery workers the least, points to an
advantage of maturity and experience. In light of recent events in
the United States, the effect for recovery workers should be interpreted
with caution. While often exposed to horror, these rescue and recovery
workers seldom experienced direct losses or extensive bereavement.
However, it is also possible that we could learn from the capacity of such
workers to support one another and to develop a meaningful narrative about
That samples from outside the United States tend to be more
severely impaired likely reflects the fact that disasters tend to be more
destructive when they occur in the developing world. Many of the samples
from developing countries survived disasters where death tolls were
measured in thousands or even tens of thousands. If this effect reflects
the importance of surviving in a context of massive destruction and death,
rather than location per se, it may have relevance for the United States
as it now grapples with the aftermath of a disaster of comparable
enormity. The difference may also attest to the ability of government
services and other resources to make a difference in the lives of disaster
Findings regarding the adverse consequences of experiencing
disasters caused by malicious human intent were unequivocal. In the
United States, technological disasters appear to be
somewhat more stressful than natural disasters. From a more global
perspective, it may be time to re-examine our ideas about the relative
impact of natural and technological events. The literature in the field
has changed markedly in the past decade. International research has
mushroomed and many of these studies have found quite severe effects. Many
of our ideas about the course of recovery from natural disasters are based
very much on western experience where predisaster housing quality,
controls over land use, and warning systems are far superior to the norms
in developing countries.
It should also be recognized that both natural and technological
disasters varied considerably in their effects, as we found examples
of low impact, moderate impact, and high impact events within each of
these categories. Few of the incidents of mass
violence had anything other than severe effects.
Overall, from these illustrative studies and others similar to them in
the database, we may conclude that disasters should have minimal
consequences for mental health at the population level beyond those
associated with transient stress reactions when:
injuries and deaths are rare
the destruction or loss of property is confined relative to the size and
resources of the surrounding community
social support systems remain intact and function well
the event does not take on more symbolic meanings of human neglect or
Such events may compose a minority of those in the published
literature, but probably a larger share of real life events in the United
States. Such events probably do not require large-scale professional or
even paraprofessional mental health interventions, although crisis
intervention strategies that ameliorate the initial stress may be helpful.
At a moderate level of impact, the typical result for major
disasters in the United States, programs that can reduce stress, enhance
social support, and provide reassurance about future risk are advisable at
the community level. Such programs might encompass mechanisms for
identifying and referring the minority of those with more serious
impairment for professional treatment.
Disasters that engender severe, lasting, and pervasive psychological
effects are rare, but they do happen. Sample (and presumably
population) level affects were greatest when at least 2 of the following
event-level factors were present:
Extreme and widespread damage to property.
Serious and ongoing financial problems for the community
Human carelessness or, especially, human intent caused the disaster.
High prevalence of trauma in the form of injuries, threat to life, and
loss of life.
When such disasters occur, the need for
professional mental health services will be widespread. Delivering them
will pose a tremendous challenge but seems to be required.
Persons who are most at risk for long-term
distress can be identified fairly early in the process, which therefore
points to a need for early screenings and interventions in disaster mental