An initial search retrieved 200 articles from PubMed, 268 from Web of Science, and 203 from Scopus for a total of 671 papers. Among these, there were 345 unique papers. Out of this set of unique papers, 89 non-human studies were excluded. These included reviews (17), case reports (3), book chapters (2), letters to editors/conference proceedings (5), and clinical guidelines (6). Twenty-six pharmacologic studies were also excluded. A total of 25 papers were excluded for having a non-respiratory health outcome, and 43 were excluded for lacking a weather-related exposure variable. Of papers that included a weather-related exposure and a respiratory health outcome, 79 were excluded as they studied a non-relevant exposure event (thunderstorm, wildfire, pollution). This left 38 potential candidate papers. A full text review of each resulted in 16 papers being included in the review. Papers spanned the globe including countries such as the US (13), Taiwan (1), and Japan (3). The sample size ranged from 58 to 715,233. Search strategy can be found in
Figure 1.
3.3. Outcomes and Study Populations
Three papers used asthma prevalence as an outcome [
49,
50,
51]. Larson et al. found that having at least one adult with asthma in the home was significantly associated with flooding with an OR of 1.42 [95% CI (1.22, 1.64)] [
49]. Eiffert et al. found that when controlling for smoking status and length of residence, self-reported current asthma was associated with higher Environmental Relative Moldiness Index (ERMI) values with an aOR of 1.12, [95% CI: (1.01–1.25); two-tailed P=0.04]. While an association between flooding and asthma was not directly compared, on bivariate analyses, homes with observed mold were more likely to have resident-reported water leaks [OR = 3.03, 95% CI: (1.51–6.08)] [
50]. Saporta et al. found that post-hurricane patients reported more asthma or lower respiratory symptoms than pre-hurricane (39% vs. 25%,
p < 0.05) [
51], but did not differentiate between asthma prevalence versus symptoms.
Four papers used a clinically significant asthma exacerbation as an outcome [
52,
53,
54,
55,
56]. Schinasi et al. found that odds of asthma exacerbation in children were 11% higher on heavy precipitation versus no precipitation days with 95% CI: (1.02–1.21) [
52]. They defined heavy precipitation days as days with >95th percentile precipitation. One study found that there was a significantly higher amount of pediatric ED visits for asthma exacerbations visits the month after a hurricane with aOR 1.81 [95% CI (1.54–2.14)], but this association was not significant when controlling for seasonal trends [
53]. Chowdhury et al. found that there were higher rates of ED visits for asthma in the two-month post-hurricane period compared to pre-hurricane (87.6 asthma patients per 1000 ED visits compared to 74.8,
p < 0.05) [
54]. A study looking at flooding secondary to combined sewer overflow (CSO) events found an increased risk for asthma-related ED visits 1 and 5 days following CSO events, with OR: 1.11 [95% CI: (0.98,1.25)] and OR: 1.12 [95% CI: (0.99,1.27)], respectively [
55]. Cowan et al. found that rates of asthma ED visits were similar in counties that received a disaster declaration and counties that did not [
56].
Seven studies used participant reported asthma symptoms such as wheezing, chest tightness, or shortness of breath [
51,
57,
58,
59,
60,
61,
62]. A study looking at home flooding found that residents of flooded homes experienced more asthma symptoms and required an increase in controller medications with aOR 3.77; [95% CI: (2.06–6.92)] and aOR 1.38, [95% CI: (1.01–1.88)], respectively [
57]. Hendrickson et al. found that primary care visits for asthma symptoms were increased in the two-week period following Hurricane Iniki with RR: 2.8, [95% CI: (1.93–4.09)] [
58]. One study found that residents of water-damaged homes had higher rates of respiratory and nasal symptoms one week after flooding with aOR: 4.19 [95% CI: (1.17–15.0) and aOR: 8.15 [95% CI: (2.39–27.8)], respectively [
59]. Cummings et al. found a positive association between exposure to water-damaged homes and lower respiratory symptoms such as cough and wheeze with
p < 0.05 [
60]. Rath et al. found that self-reported upper-respiratory symptoms (nasal congestion) and lower-respiratory symptoms (coughing and wheezing) were higher after a hurricane (76% and 36%, respectively) than before the hurricane (22% and 9%, respectively) with
p < 0.0001 [
61]. Another study demonstrated that 9.3% of their participants reported worsened asthma symptoms after a typhoon, and an increased proportion of participants received systemic corticosteroids as a rescue medication over the study period of 1 year (47.0% vs. 19.5%,
p = 0.033) [
62]. Of note, it is standard of care to treat an asthma exacerbation with systemic corticosteroids, but data on asthma exacerbation frequency were not collected in this study.
Two papers studied COPD. Qu et al. used COPD exacerbations requiring hospitalization as their outcome and found that higher rates of COPD hospitalization were associated with major storms, which mainly included flooding, thunder, hurricane, snow, ice, and wind across a lag of 0–6 days. Adjusted RRs ranged from 1.23–1.49, with significant effects on lag days 0–4 [95% CI: (1.05–2.58)] [
63]. Shih et al. found that among adults with cerebrovascular and cardiac disease, patients with chronic obstructive pulmonary diseases (COPD) and asthma had significantly increased mortality rates after a typhoon, with adjusted HR 1.7–2.1 [
64].
No studies on AR that fit the search criteria were found. Papers reviewed for inclusion on AR had average daily precipitation as their exposure available [
65], which did fit the criteria of extreme weather.
Study population ages and locations were varied among papers. Four papers studied children [
52,
53,
55,
61]. Brokamp et al. defined cases as aged 0–18 years who visited the Cincinnati Children’s Hospital Medical Center between January 2010 and December 2014. Fanny et al. selected individuals under the age of 18 who were seen at pediatric emergency departments and urgent care centers in Houston, Texas one year before and one month after Hurricane Harvey. Rath et al. included children and adolescents below the age of 24 residing in New Orleans immediately following Hurricane Katrina; 90% of their sample was age 15 or below [
61]. Schinasi et al. studied the population of children in Philadelphia, PA aged 0–18 years.
Four studies included children and adults [
49,
51,
56,
58]. Cowan et al. included all patients who visited a hospital for asthma after Hurricane Irene, ranging from age 1 to 101; 44% of participants were under the age of 18 [
56]. Hendrickson et al. included all Kauai residents who sought outpatient, inpatient, or emergency care after Hurricane Iniki. The study population in Larson et al. was comprised of Detroit, MI households. Saporta et al. sought to represent the population of Northern New Jersey impacted by Hurricane Irene and Sandy. Their pre-hurricane group and post-hurricane group had 12% and 29% of participants below the age of 18, respectively [
51].
Six papers studied adults [
50,
57,
59,
60,
62,
63,
64]. Azuma et al. chose adults who experienced flooding secondary to typhoons or heavy rainfall in six Japanese cities between 2004 and 2010. Chowdhury et al. studied the population of St. Thomas in the US Virgin Islands impacted by Hurricane Irma and Maria and did not specify whether they included an adult or pediatric population. Cummings et al. included adults residing in New Orleans in March 2006. Eiffert et al. selected adults living in the English Avenue and Vine City neighborhoods of Atlanta in the Proctor Creek watershed. Hoppe et al. included individuals affected by the 2008 Cedar Rapids flood in Iowa. Qu et al. selected adults in New York State hospitalized with a primary diagnosis of COPD. Sato et al. sought to study asthma patients in Japan during typhoon season; while their study population was not directly specified, at least 95% of their sample was above the age of 18 based on mean and standard deviation [
62]. Shih et al. studied the adult population of Taiwan affected by Typhoon Morakot who carried a diagnosis of acute ischemic heart disease, intracranial hemorrhage, or ischemic stroke.
One study in Japan on typhoons and asthma exacerbations studied a population of persons with asthma who were currently undergoing treatment at the time of study [
62]. They excluded patients who had upper or lower respiratory infections or who had changed their treatment regimen during the study period.
3.4. Case Definitions
Asthma exacerbations are defined by having reduced expiratory airflow function along with one or several of the following symptoms: shortness of breath, cough, wheezing, or chest tightness [
66]. The Fuhlbrigge reference appears to be a standard definition. One paper used this definition for their cases [
52].
However, many papers did not use a standard definition, and simply asked participants to self-report asthma symptoms. Some papers [
59,
60] asked participants to report their asthma symptoms in an interview or questionnaire. Other papers [
50,
57] asked participants to report any asthma symptoms and whether they had received a diagnosis of asthma from a doctor in the past. Larson et al. asked participants if they or members of their household carried a diagnosis of asthma [
49]. Sato et al. conducted a patient interview, and recorded data on participants’ asthma symptoms and whether they had needed a systemic corticosteroid as a rescue medication (a proxy for asthma exacerbation). They defined worsening symptoms as “a condition where a subject has become aware of at least one symptom change or took reliever medication” [
62]. One study used data from the Health Survey for Children and Adolescents After Katrina survey and examined self-reported upper and lower respiratory symptoms, asthma diagnosis, and asthma attacks [
61]. Saporta et al. conducted a chart review of patients reporting asthma symptoms [
51].
Other papers conducted a record review using ICD codes. For studies using asthma as a health outcome, most used the ICD-9 493.XX definition, while others used the ICD-10 system with J45 as their diagnosis code. Brokamp et al. defined cases as ED visits with a primary diagnosis using ICD-9 [
55], while Chowdhury et al. defined ED visits with the ICD-10 classification [
54]. One paper used both the ICD-9 and ICD-10 systems [
52]. Cowan et al. conducted a record review of ED visits and inpatient visits with asthma as the primary diagnosis using ICD-9 [
56]. Fanny et al. also reviewed ED cases and stated they used ICD-9 and ICD-10 systems to identify asthma cases, but did not specify exact codes [
53]. Another study reviewed records from EDs, inpatient admissions, and outpatient clinics in their area with a primary diagnosis of asthma using ICD-9 [
58]. Shih et al. reviewed both outpatient and inpatient records and defined asthma cases using ICD-9, and also defined COPD cases using the ICD-9 codes 491–492, 494, and 496. They counted mortality cases if a patient’s record was withdrawn from the Taiwan NHI insurance database due to death [
64]. One study using COPD as their health outcome reviewed inpatient records and chose cases where the COPD ICD-9 codes 491, 492, and 496 were listed as the principal diagnosis for admission [
63].
3.6. Statistical Analysis
Sato et al. had a small sample size and conducted simple Chi-square,
t-tests, and Fisher’s exact tests for differences in exposure variables such as pollen concentrations between persons whose symptoms worsened following typhoons and those whose symptoms did not worsen [
62]. They also compared these groups using a logistic regression model including multiple measures of IgE antibodies and rescue us of systemic CS for the past year.
Another study from Japan compared the health impacts of flooded and non-flooded homes using Chi-square tests for bivariate associations. Researchers used multivariate models to compare flooded and non-flooded homes controlling for sex, age, and household factors. Though the study was conducted by sampling from several different sites, no attempt was made to include regional random effects [
59].
The study of asthma-related ED visits from North Carolina compared counties impacted by Hurricane Irene with those that were not. They used a difference of differences analysis conducted on a log scale using a Poisson generalized linear model to estimate the impact of the hurricane on counts of ED visits. This model controlled for the count, month, and year. They took spatial factors into account (correlation between counties) using an autoregressive-1 correlation structure [
56].
Some studies used systematic sampling strategies. A study from New Orleans selected residential census tracts at random and then randomly assigned waypoints within selected tracts to select households. When persons refused or were not home, teams went to another household using an unspecified criterion [
60].
The study from New Orleans created a scoring system for both exposures and outcomes. Exposures were measured through a “clean up score” equal to the sum of the reported number of homes cleaned that had less than 50 percent of walls and ceilings covered with mold. From there, they categorized this score along with other contextual information. For outcomes, they used a composite score of asthma symptoms as measured by the survey instrument [
60].
3.7. Exposure Assessment
For extreme storm exposures, several methods were used. Schisani et al. defined extreme precipitation as storms with greater than the 95th percentile of precipitation for the year using data from NOAA and controlling for temperature [
52]. Qu et al. obtained major storm dates and locations obtained from the Integrated Surface Database of the U.S. National Oceanic and Atmospheric Administration [
63].
Chowdhury et al. studied hurricanes as their exposure variable. The ED evaluated in the study is located in the only hospital on St. Thomas, and it was assumed all patients presenting to the ED were affected by hurricanes Irma and Maria [
54]. An additional paper with an island population assumed all residents of Kauai were exposed to Hurricane Iniki [
58]. Another paper studying hurricanes took patients’ billing addresses from ED records and considered patients who lived in a county that received a FEMA disaster declaration from Hurricane Irma as exposed [
56]. Fanny et al. assumed the entire population of Houston was exposed to Hurricane Harvey as one-third of the city flooded but did not collect individual exposure data [
53]. Another paper chose their non-exposed vs. exposed group by identifying patients in the same clinic who were evaluated before and after Hurricane Harvey, respectively [
51].
Sato et al. assumed all Fukushima residents were exposed to the 2013 typhoons [
62]. Another paper chose patients for their study who were designated as living in an affected area of Typhoon Morakot by the Taiwan National Health Insurance (NHI) Database [
64]. The NHI had identified persons living in affected areas in order to provide financial help for their medical expenses.
Papers that identified flooding as their extreme weather event used similar methods. Azuma et al. obtained information from the Tokyo Ministry of Land, Infrastructure, Transport and Tourism to identify severely flooded areas, and then worked with local public health centers to identify households with water-damaged homes [
59]. Another paper obtained the locations and dates of combined sewer overflow events from the Metropolitan Sewer District (MSD) of Greater Cincinnati; a household was considered exposed if it was within 500 m of an overflow flood event [
55]. Hoppe et al. designated homes as water-damaged by the Cedar Rapids flood if they were marked for follow-up by the Code Enforcement Division of Cedar Rapids [
57].
Other papers used participant questionnaires to determine exposure to flooding. Cummings et al. conducted interviews on water damage including the extent of walls and ceilings covered in mold [
60]. A study on urban pluvial flooding identified houses where the house had flooded from the outside with water covering at least a quarter of the floor of a room [
50]. A similar study used participant questionnaires on whether flooding had occurred in the home since they had lived there [
49]. Rath et al. used participant-reported flood damage as their exposure assessment [
61].
3.8. Results of Individual Studies
Of the 16 papers that were selected for review, weather events included flooding, heavy precipitation, hurricane, and typhoon. Non-communicable respiratory conditions included asthma and COPD.
For studies evaluating a flooding event, six papers found significant association between home flooding and asthma [
49,
50,
55,
57,
59,
60]. Qu et al. [
63] found a significant association between major storms, which included flooding, and COPD. However, this study did not differentiate between different types of storms in their analysis.
Four papers found significant associations between hurricanes and asthma symptoms [
51,
54,
58,
61]. Qu et al. found an association between COPD hospitalizations and major storms but did not differentiate the type of storm in analysis [
63]. Cowan et al. found no association between asthma ED visits in counties affected by a hurricane (counties that received a disaster declaration) and those who did not [
56]. Fanny et al. found a significant association between hurricanes and pediatric asthma exacerbations, but this association was not significant when accounting for seasonal trends [
53].
One study a found significant association between typhoons and asthma exacerbations requiring steroids [
62]. However, this study had a small sample size and did not adequately control for confounding factors in their analysis.