I have been commuting regularly by Grab and Uber cars for the past 11/2 years and have found that some of these vehicles, especially the older rented ones, tend to emit a musty smell from the air-conditioners. When I brought up the issue with some of the drivers, I was informed that the filters in the air-conditioners have gone mouldy as a result of poor maintenance. Some of the drivers resorted to using air fresheners, but this does not get to the root of the problem.
As a safety precaution, I can only wind down the windows. The inhalation of mould particles is bad for health, regardless of whether the driver or passenger is asthmatic. In fact, it will be worse for an asthmatic person. Having also commuted by taxis for many years, I must say that taxi companies here do a good job in maintaining the overall condition of their fleets. It is time private-hire car companies strove to do better in terms of the overall cleanliness - particularly the air quality - of their cars.