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Heat Health Events (HHE)

California is already warmer and it is not a secret of how things might turn warmer over the next century. More frequent and severe heat events will pose considerable health risks to the communities and to vulnerable populations.

There is growing evidence that the character of heat waves in California is also changing. Heat events are becoming progressively more humid, lasting longer, and also occurring in areas not accustomed to heat waves.

While some studies have shown a lower rate of heat-related mortality in recent years, in part due to higher prevalence of air conditioners and implementation of heat awareness and mitigation programs, more severe and prevalent heat waves will expose California residents, particularly vulnerable groups, to health risks.

What makes matters worse is that, climate change will also challenge the efficacy of traditional intervention strategies, and local agencies may struggle to effectively mitigate heat-health impacts.

California recently began the state’s Fourth Climate Change Assessment, a state-mandated research program to assess climate change impacts in California. Better understanding of the public health impacts of climate change is one of the state’s identified priorities. This research project was undertaken with intent to build a decision-support tool informed by decision makers tasked with reducing the long-term public health impacts of extreme heat.

Climate change threatens health in a myriad of ways, including increases in vector- and water-borne diseases, decreases in air and water quality, and impacts from more extreme weather events such as droughts, floods, and hurricanes. One of the most apparent health risks stemming from climate change is increasingly frequent and longer periods of more severe extreme heat.

Coastal population; Income; The elderly and Urbanisation have been identified as the vulnerable groups and regions.

Humidity; Night-time temperature; The Urban Heat Island Effect - UHI and Poor air quality are the major phenomena affecting health.

Which brings us to the question - Are there any heat criteria & thresholds?

Unfortunately, there is still no consistent definition or method for identifying heat wave thresholds, specifically with regards to impacts on human health. To assess the severity of an extreme heat independent of its impact on people could be dangerous, and definitions based on aspects of human health may offer a more accurate basis for planning and preparedness.

As city, county, and state planners think through policies and programs that can address heat related risks, they must first understand what heat thresholds will be dangerous for the

populations who live in affected areas. These are termed as thresholds baselines. Unless these baselines are people focused, meaning they evaluate heat risks from a public health perspective, then existing risk thresholds may lead to misinformed long-term planning.

Heat Health Events (HHE)

A heat-health event (HHE) can be characterised by a set of meteorological conditions occurring over a period of days that, historically, has been associated with significant negative public health impacts. These events exceed thresholds of human response to heat, as defined by the heat-health (i.e., exposure-response) relationships at which negative health outcomes occur. This framework and new dataset is intended to serve as a baseline for heat adaptation planning efforts when integrating climate change.

Local areas are consolidated with similar heat and humidity characteristics into heat wave zones (HWZ), roughly the size of a medium-sized city.

Since vulnerable populations (infants, elderly, certain racial groups) are typically more

sensitive to excess heat, two sets of HHE definitions for each HWZ are identified, one for lower risk individuals and another for higher risk individuals.

Furthermore, acclimation of the human body evolves over the summer, and it is found that heat has different impacts on health in the same area from month to month. Accordingly, the HHE definitions categorised to reflect acclimation of local population by month, grouped in

three seasons with similar characteristics: April-May, June-July-August, and


Given rising average temperatures and increasingly frequent and prolonged heat waves, developing climate and population-specific thresholds may offer a more adequate basis for climate change adaption planning. HHE thresholds may be especially useful for seasonal- and population-specific heat wave planning.

While adequate thresholds alone do not prevent heat-related illnesses and death, locally

relevant heat-health thresholds and contextual information can help support officials in

sustainability, housing, transportation, urban planning and public health.

The research hopes that the use of medically-informed baselines to project future HHEs may

provide a more adequate basis for evaluating future health risks with the use of climate projections.


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