Warmer air is less dense. I don't know if there would be enough difference during heat waves for this to be noticable.

http://www.eurekalert.org/pub_releases/2016-01/dai-ldh012216.php

Public Release: 22-Jan-2016
Lung disease: Higher mortality during heat waves
Deutsches Aerzteblatt International

Heat waves are becoming more common in summer and have health-related consequences. In patients with chronic lung disease, the risk of dying increases by up to 43%, according to an original article by Christian Witt and colleagues in the current issue of Deutsches Ärzteblatt International (Dtsch Arztebl Int 2015; 112: 878-83). The probability that a patient with chronic lung disease develops additional symptoms increases by some 9% during a heat wave.

In the scientists' view, this is due to a combination of heat stress and noxious/toxic substances, such as greenhouse gases, fine particle dust, or ozone. The possible outcome: acute and chronic lung damage. In patients with lung disease, each hot day brings clear risks: the daily probability of dying from the consequences of the high temperature increases by up to 3.4% -- and that is the case for each degree Celsius (1.8 degree Fahrenheit) that the climate rises above the mean summer temperature.

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