The human body operates at an average internal temperature of 37°C, give or take various fluctuations during the day. But too much or too little external heat can exacerbate certain health conditions. Chronic obstructive pulmonary disease (COPD), a group of lung diseases that includes bronchitis and emphysema, has been linked to higher temperatures, possibly triggered by inflammation or oxidative stress – an imbalance between the body’s production of free radicals and its ability to neutralise their harmful effects with antioxidants. One study also linked cold weather with an increased risk of heart attack.

But understanding what a 1°C change in air temperature means isn’t that easy. A degree change in a warm climate, for example, is different to the same change in a cold climate. Humidity, air pressure, wind speed, cloud cover, and radiation reflux could all also have an impact on the body since they strongly interact with air temperature in the environment. Quite a few studies have reported conflicting results because health professionals haven’t considered all of these factors. Instead, meteorologists have proposed using calculations called the physiological equivalent temperature index or the universal thermal climate index, which include all relevant meteorological parameters to model the weather.

In a recent investigation I carried out with Andreas Matzarakis, a meteorology professor at the University of Freiburg, we found that the largest difference between air temperature alone and the calculation made using the physiological equivalent temperature index in Australia was in the summer time. However, in the northern hemisphere the situation appeared to be the opposite, with the biggest difference in the winter time. Studies have also been carried out in countries such as Austria, Germany, Greece, Slovenia and Taiwan.

Using these calculations can help make better comparisons between different populations living in different climates and is already showing some interesting results in respiratory and cardiovascular problems. Using both the indexes, we recently showed that lower temperature was associated with coronary heart disease and hypertension, or high blood pressure. These indexes can also be used to identify periods of cold stress, which could help predict and relocate medical resources such as doctors, nursing time, beds and ambulances. For example, there were six more admissions due to coronary heart disease and eight more for hypertension observed on a cold stress day in Plymouth, compared to a non-cold stress day.

Imagine if every year there were 165 days that were classified as cold stress days, that would amount to up to 990 excess admissions due to coronary heart disease and 1,320 excess admissions of people with hypertension. Suppose each extra admission required at least 15 min for medical attention in the first instance, that would mean up to 278 hours extra that need to be scheduled for diagnosing and admitting patients with coronary heart disease and 330 hours for hypertension. Like other examples in epidemiology, smaller numbers begin to add up.

When it comes to indoor environments, nearly 5% of cases of high blood pressure in Scotland could be prevented by maintaining indoor temperature above 16°C, and 9% could be prevented if the indoor temperature could be maintained above 18°C.

Many cardiovascular disease patients, particularly stroke patients, have been found to have a blood pressure surge in the morning, and it is probably due to the lower temperature outside and in the home. Japanese researchers found that intensively heating a room in the winter decreased the morning blood pressure surge. However, this could be a costly solution for most people, especially in the deprived areas, and government schemes to promote energy saving and better building design might help.

In addition, researchers from the US found from animal research that there could be a novel way to block the development of hypertension, or high. Cold exposure led to an associated increase in activity in the sympathetic nervous system, which helps run body organ functions, and the production of nitric oxide in rats. The addition of a nitric oxide synthase inhibitor, a chemical, to their drinking water reduced the intensity of the effect in cold-exposed rats. If this experiment could be replicated in humans and the same effect confirmed, this might be a new breakthrough for human health.