Exposure to flood ups risk of death

Communities that are prone to floods appear to have increased risks of all-cause, cardiovascular, and respiratory deaths that can last for up to 60 days, reveals a time series study. Such associations could vary depending on local climate, socioeconomic status, and older age.

“The increases in risk of mortality were most prominent around 25 days after exposure to floods and lasted for up to 60 days,” the researchers said. “The associations appeared to be modified by climate type and were strongest in communities with a low socioeconomic status and a high proportion of older people.”

A total of 761 communities in 35 countries or territories, with at least one flood event during the study period, were included in the analysis. The following databases were accessed to obtain relevant information: Multi-Country Multi-City Collaborative Research Network, Australian Cause of Death Unit Record File, New Zealand Integrated Data Infrastructure, and the International Network for the Demographic Evaluation of Populations and their Health Network.

The researchers modelled an estimation for the lag-response association between flood and daily mortality risk, and they calculated the overall effects by gathering the relative risks (RRs) over the lag period. They also calculated attributable fractions of mortality caused by floods. Flooded days referred to days from the start to the end date of flood events.

Additionally, the research team examined how daily death risk correlated with flooded days in each community using a quasi-Poisson model. They also pooled the community-specific associations through random effect multivariate meta-analyses.

Overall, 47.6 million all-cause deaths, 11.1 million cardiovascular deaths, and 4.9 million respiratory deaths were included in the analysis. Mortality risks rose and persisted for up to 60 days (50 days for cardiovascular death) after a flooded day over the 761 communities. [BMJ 2023;383:e075081]

The cumulative RRs were 1.021 (95 percent confidence interval [CI], 1.006‒1.036) for all-cause mortality, 1.026 (95 percent CI, 1.005‒1.047) for cardiovascular mortality, and 1.049 (95 percent CI, 1.008‒1.092) for respiratory mortality.

Notably, the association between death and exposure to flood varied across countries or territories and regions. Likewise, these associations were modified by climate type and were more evident in poor communities and in populations with a low human development index or high share of older adults.

In places affected by flood, up to 0.10 percent, 0.18 percent, and 0.41 percent of all-cause, cardiovascular, and respiratory deaths were attributed to floods, respectively.

“Policymakers and health professionals should raise awareness of the increased mortality risk after floods to improve disaster response strategies and thereby reduce the number of avoidable deaths,” the researchers said.

Counterintuitive

On the other hand, a study on all floods in England and Wales from 1994 to 2005 revealed an inverse association between flood and mortality. The results showed a 10-percent decrease (95 percent CI, 0‒18) in the risk of all-cause death in flood areas after events. [Environ Health 2011;10:11]

“This is consistent with our findings in the UK,” the researchers said. “Similar counterintuitive associations were also observed in Thailand, Portugal, and the Philippines.”

Possible reasons for this phenomenon were as follows: first, some people’s deaths were not registered at their usual place of residence because of evacuation; second, exposure to flood raised the level of attention to personal health and health services.

“Local death registration systems and disaster response policies should be considered when interpreting flood-mortality associations,” the researchers said.