However, findings related to age and gender have been inconsistent so far, so researchers from the University of Toronto, Canada, started to examine the possible link between summer high temperatures at night and increased deaths amongst people aged 60 and 69 years due to heart diseases.
They studied data from the Office for National Statistics on adult deaths attributed to CVD for June and July every year between 2001 and 2015 in England and Wales because heatwaves in the UK are most frequent and intense during these months.
They also gathered corresponding information from official USA data for King County, Washington, a similarly sea-facing region, at parallel latitude to England and Wales, with comparable land-ocean atmospheric properties and similarly low prevalence of residential air conditioning. The US data, however, only included men.
Results showed that between 2001 and 2015, there were 39,912 CVD deaths (68.9% men) recorded in England and Wales and 488 deaths in King County.
In England and Wales, after allowing for certain variables, a 1°C rise in the usual summer night-time temperature was associated with a 3.1% increase in the risk of CVD mortality among men aged 60-64, but not older men or either women age groups.
In King County, a 1°C rise was associated with a 4.8% increased risk of CVD mortality among those aged 65 and under, but not in older men.
Over the 15 years observed, heart disease rates overall declined substantially in both regions annually and notably over the summer months, in line with greater population uptake of effective primary and secondary preventive therapies over time.
This was worrying, they added because, in recent years, populous regions such as the ones studied had experienced a proportionate rise in night-time rather than daytime summer heat intensity.
This is an observational study, so can’t establish causality, and the researchers acknowledge some limitations to their work such as the unavailability of weekly outcomes and exposure data by district or city level.
The present findings should stimulate a similar investigation of exposure and event rates in other populous mid-latitude to high-latitude regions.
Considering the growing likelihood of extreme summers in Western USA and UK, these results invite preventive population health initiatives and novel urban policies aimed at reducing future risk of CVD events.