Heat wave threshold ignores public health impact

The first of its kind Indian study looking at increased mortality due to heat waves provides valuable data that can reset the way precautionary steps are taken to reduce deaths by keeping in mind the public health priorities. A paper published in March last year in the journal PLOS ONE is based on the excess all-cause mortality associated with the 2010 Ahmedabad heat wave.

Ahmedabad witnessed an additional 1,350 deaths during the heat wave of May 19-25, 2010, which included an extreme heat wave condition for four days. This represents 43 per cent increase in deaths compared with the corresponding period in 2009.

The Indian Meteorology Department (IMD) defines heat wave as one where there is either an excess of 5 to 6 degree C over a normal daily historical maximum temperature (over a 30-year period) of less than 40 degree C or an excess of 4 to 5 degree C over a normal historical maximum temperature of more than 40 degree C.

However, immaterial of the normal historical maximum temperature, IMD declares heat wave when the actual maximum temperature is above 45 degree C.

“The IMD declares a heat wave based on arbitrary temperature cut off values — a procedure that was developed long ago —and has no public health basis whatsoever,” said Prof. Dileep Mavalankar a co-author of the paper from the Delhi-based Public health Foundation of India and Head of the Indian Institute of Public Health, Ahmedabad.

Considering the fact that many hundreds die due to heat waves in some city or region of India almost every year, there is an overwhelming need is to arrive at optimal definitions of a heat wave from the public health perspective.

“Our findings suggest that the IMD definitions may underestimate the impacts of extreme heat on health because under the current systems the IMD threshold does not formally account for public health effects of extreme heat. For public health purposes the IMD definitions may not be as useful as a definition with lower thresholds that are observed to correlate with public health outcomes more directly,” they note.

Thanks to urban heat island effect, temperature in cities and urban areas are on average 3.5 to 12 degree C higher than rural areas and areas outside the city. But as was seen in the case of Ahmedabad in 2010, the IMD temperature monitoring station was located on the outskirts of the city. As a result, the true maximum temperature in the city due to urban heat island effect is not measured and appropriate warning comes either too late or is never issued.

The study has not been able to take into account the actual indoor temperature in the case of those who died indoors. It is unclear if the ambient daily temperature measurements are an under- or over-estimate of the indoor temperature, the authors note.

Another concern is the heightened possibility of underreporting of deaths caused due to heat wave. Deaths taking place at homes and in those who are homeless are unlikely to be reported they write.

“The day’s death record should be published the very next day,” demands Prof. Mavalankar. “Now birth and death registrations are largely for administrative purposes and not for public health purposes. This should change.”

Published in The Hindu on June 11, 2015