Your query is about which floor would be ideal for healthy living. I do not think the floor on which the flat is would determine whether or not the residents in it would have a healthy life! Surely one's lifestyle, nutrition and taking care of one's health generally would determine this.
You mention a 21-storey building. In that case, it is highly likely that the builder would have made provisions for parking. Nowadays parking options have increased, with not just spaces in the compound or the basement but even tiered multi-leval parking with the flats starting at a higher level. Hence, if one is residing on the floor immediately above, one is likely to be disturbed by the constant going and coming of vehicles. Those who are totally averse to noise should then opt for at least the second floor or higher.
With regard to higher floors, especially above the 10th floor, I have had input from those who bought flats on the 14th and 18th floors about the horrendous rattling of windows and doors, even when tightly shut, during strong winds, especially during the monsoon season. Just the noise of the howling wind and the heavy rain is also unbelievably high. One person even told me how the window's glass cracked because of the wind's velocity. Then there is the problem of the lift breaking down. If there is no back-up automatic generator, then it will be a major problem. It is very tiring for most people to climb up stairs beyond, say, the fifth floor, and if there are senior citizens, then it is really tough. Most important are the difficulties faced during an emergency, such as when there is a fire. Even if the building has received a green signal from local fire officials, there is the possibility that the installed fire system does not function due to non-maintenance or a sudden break-down. One is not supposed to use the lift at such times due to the possibility of being trapped in it. There may be other such possible hazards which one should consider before selecting a flat on the upper floors.
Basically, the residents of upper floors need to keep in mind that while they have the advantage of a superb view and are away from most noises as well as less likely to have a mosquito menace if this is common in the location, there are more disadvantages actually.
Looking beyond the floor level, there are other dimensions as well. For example, one could also consider whether the flat has front-facing windows or these open on the rear or corner/sides. I know of people who, when they went house-hunting, specifically told the real estate broker that they would like a front-facing flat so that when they looked out they could see the main road and people around, thus not feeling that they were totally cut off.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell