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Summary: Background: Epidemiological evidence on short-term association between ambient carbon monoxide (CO) and mortality is inconclusive and limited to single cities, regions, or countries. Generalisation of results from previous studies is hindered by potential publication bias and different modelling approaches. We therefore assessed the association between short-term exposure to ambient CO and daily mortality in a multicity, multicountry setting. Methods: We collected daily data on air pollution, meteorology, and total mortality from 337 cities in 18 countries or regions, covering various periods from 1979 to 2016. All included cities had at least 2 years of both CO and mortality data. We estimated city-specific associations using confounder-adjusted generalised additive models with a quasi-Poisson distribution, and then pooled the estimates, accounting for their statistical uncertainty, using a random-effects multilevel meta-analytical model. We also assessed the overall shape of the exposure–response curve and evaluated the possibility of a threshold below which health is not affected. Findings: Overall, a 1 mg/m3 increase in the average CO concentration of the previous day was associated with a 0·91% (95% CI 0·32–1·50) increase in daily total mortality. The pooled exposure–response curve showed a continuously elevated mortality risk with increasing CO concentrations, suggesting no threshold. The exposure–response curve was steeper at daily CO levels lower than 1 mg/m3, indicating greater risk of mortality per increment in CO exposure, and persisted at daily concentrations as low as 0·6 mg/m3 or less. The association remained similar after adjustment for ozone but was attenuated after adjustment for particulate matter or sulphur dioxide, or even reduced to null after adjustment for nitrogen dioxide. Interpretation: This international study is by far the largest epidemiological investigation on short-term CO-related mortality. We found significant associations between ambient CO and daily mortality, even at levels well below current air quality guidelines. Further studies are warranted to disentangle its independent effect from other traffic-related pollutants. Funding: EU Horizon 2020, UK Medical Research Council, and Natural Environment Research Council.

Background: Floods are the most common type of natural disaster globally. Climate change and the pressures of development to accommodate increasing population sizes could increase the number of people at risk of flooding. Although the immediate risk of death and injury from flooding is small in high-income countries, reports suggest that flooding has prolonged adverse effects on mental wellbeing. However, evidence is scarce because most previous studies have used surveys or self-reports, without robust pre-flood records. This methodological limitation applies to other natural and man-made disasters. Methods: We investigated the mental health effects of floods in England using routine prescriptions data from pre-flood and post-flood periods in flooded and non-flooded populations. We examined prescription records for drugs used in the management of common mental disorders from 930 primary care practices located within 10 km of large flood events in England in 2011–14. We conducted a controlled, interrupted time-series analysis of the number of prescribing items for antidepressant drugs in the years before and after the flood. Pre–post changes were compared by distance of the practice from the flooded boundaries. Analyses were controlled for neighbourhood deprivation and population density or the number of prescriptions for non-antidepressant drugs. Findings: After controlling for neighbourhood deprivation and population density, prescriptions for antidepressant drugs among practices located within 1 km of a flood increased by 0·59% (95% CI 0·24–0·94) in the year after the flood compared with practices furthest (5–10 km) from the flood. The results did not change after controlling for non-antidepressant drugs. The increase in prescriptions was greater in more socioeconomically deprived than in less socioeconomically deprived areas. Interpretation: We found that the number of prescribing items for antidepressant drugs increased in the year after a flood in primary care practices close to major floods in England in 2011–14. This small percentage increase would, if caused by flooding, reflect a much larger increase in flooded households, which made up only a small proportion of the total number of households registered with these practices. Moreover, despite the increase being small in relative terms, it, together with the well-known high prevalence of mental disorders in the UK, suggests a substantial mental health burden after flooding, which has implications for public health practices. More detailed linkage studies are required to disentangle the effect of flood exposure on individual patients from the observed increase in prescriptions for antidepressant drugs at primary care practices. We are currently extending this study to include prescribing data for individual patients in primary care practices. Funding: Economic and Social Research Council (ES/L007517/1) and the National Institute for Health Research, Health Protection Research Unit in Environmental Change and Health.