Background: Homelessness represents a widespread social issue globally, yet the risk of neurodegenerative diseases (NDDs) associated with people experiencing homelessness (PEH) has not received sufficient attention. Therefore, this study aimed to explore the risk of NDDs among PEH and its variation across countries and regions through meta-analysis and systematic review. Methods: Searching from databases such as PubMed and Web of Science, relevant research articles on PEH and NDDs were identified. After multiple screening, eight articles were selected for meta-analysis. Statistical methods and models were used to evaluate the association between PEH and NDDs, stratified by disease type and country. Results: We found that PEH had a 51% higher risk of NDDs (OR = 1.51 (95% CI: 1.21, 1.89)) compared with those with stable housing. Specifically, PEH had a significantly higher risk of developing multiple sclerosis (OR = 4.64 (95% CI: 1.96, 10.98)). Alzheimer’s disease and related dementias (ADRD) (OR = 1.93 (95% CI: 1.34, 2.77)), dementia (OR = 1.69 (95% CI: 1.26, 2.27)), and cognitive impairment (OR = 1.07 (95% CI: 0.98, 1.16)) were all at higher risk. Furthermore, country and regional differences were observed, with countries such as Iran (OR = 4.64 (95% CI: 1.96, 10.98)), the Netherlands (OR = 2.14 (95% CI: 1.23, 3.73)), the United States (OR = 1.66 (95% CI: 1.25, 2.22)), and Canada (OR = 1.06 (95% CI: 1.01, 1.10)) showing a higher risk of NDDs among the PEH. Conclusions: The study emphasizes the significant NDD risks among PEH, providing novel perspectives on this issue and shedding light on national disparities influenced by variations in healthcare systems and social environments. This will be beneficial for academia and government to prioritize the health of PEH with NDDs, aiming to mitigate disease incidence and economic burdens while preserving social stability and upholding basic human rights.

The negative impact of fire smoke on human health is widely recognized. However, studies have not adequately compared subtle differences in rates of emergency department (ED) visits and hospital admissions (HA) for various diseases, nor have they grounded comparisons in differences in healthcare systems across countries. To bridge this gap, we conducted a systematic meta-analysis to assess global health risks associated with fire smoke exposure. We performed an up-to-date systematic review by searching PubMed and Web of Science databases, using ED visits and HA as the main hospital indicators, and meta-analyzed the estimated risks. Our findings revealed an overall odds ratio (OR) of 1.07 (95 % CI: 1.06, 1.07) for global health risks (7 % increase) post-exposure to fire smoke. Specifically, the OR increased by 9 % for ED visits (OR = 1.09; 95 % CI: 1.08, 1.10) and by 4 % for HA (OR = 1.04; 95 % CI: 1.03, 1.05). When analyzing data by country, we noted that fire smoke exposure was associated with heightened disease risk in Sweden (OR = 1.63; 95 % CI: 1.22, 2.17), Canada (OR = 1.27; 95 % CI: 1.10, 1.46), the United States (OR = 1.10; 95 % CI: 1.08, 1.11), Brazil (OR = 1.07; 95 % CI: 1.04, 1.10), and Australia (OR = 1.04; 95 % CI: 1.03, 1.04). By continent, Europe exhibited the highest risk increase at 63 %, followed by North America at 10 %, South America at 7 %, and Australia at 4 %. By disease type, the most prevalent conditions were respiratory diseases, with a 7 % increase for total respiratory diseases and chronic obstructive pulmonary disease (COPD), 5 % for other pulmonary diseases, and 11 % for asthma. Cardiovascular diseases also showed significant increases: 5 % for general cardiovascular issues, 18 % for heart failure, and 5 % for ischemic diseases. Additional health concerns included headaches (26 %), itching (28 %), and skin allergies and rashes (34 %). We also explored the toxicological mechanisms behind these disease risks, the impact of climate variations, and the disparities in healthcare systems across different countries. This study provides stakeholders such as academics and physicians with a systematic understanding of the quantifiable health risks associated with fire smoke exposure. Additionally, it innovatively compares the differences between ED visits and HA, and between environmental and health systems in each country. The results of this study provide a foundation for subsequent relevant academic research and offer easy-to-understand information for policymakers, healthcare organizations, and the public. Ultimately, this will contribute to a better understanding and mitigation of the health risks associated with fire smoke.