I live in a section of the country that is extremely humid in the summer. The only outdoor activity I can think of that would be pleasant in such circumstances is swimming. If you are going to be moist, you might as well be completely moist! :-)
You may want to look online for an air quality report. In Los Angeles, the major newspapers publish air quality reports along with the weather so that people can get some idea of whether or not prolonged outdoor activity is a good idea.
I'd expect Hong Kong to do this too.
I don't think you should be outdoors in the humidity if you are not used to it. For example, a vacationer. It depends on the humidity in the area normally. I also suggest that people with help problems take it easy on physical activity in humid weather.
If you are participating in outdoor activities in weather that your body is generally accustomed to, you should be fine. It is important to stay hydrated, but it is also important to pay attention to your bodies signals. If you feel faint, weirdly hot, or can't catch your breath, you need to slow down and cool off.
The important thing is that if you are going to be out in high heat and humidity (wherever you are), that you should consume lots of liquids. This obviously helps avoid the risk of dehydration, and it also helps keep the body from overheating.
I don't think it matters if it's in Hong Kong or Florida. You still have to look at the combination of heat and humidity. The humidity makes it hard to cool off because your sweat doesn't evaporate so easily.
A humidity of 80% is really not that high for Central Florida, where I live. If the temperature is relatively moderate, such as 80 degrees, this makes for a great day for outside activities. However, as the temperature and humidity rise, the threat of dehydration and heat stroke also goes up. In Florida's summer heat, when the temperature approaches 100 degrees with 90%+ humidity, it becomes much more dangerous, especially for people who are in less than perfect health.
Excuse me, as where I live we use degrees celcius.
It is generally not acceptable to do extreme or rigorous activities outside with high humidity (75% or above) and high heat (35 degrees celcius or higher). However, below those margins, it is generally acceptable, provided one brings plenty of cold water, towels if necessary, a mobile phone with credit and a buddy or two for safety.
But if it is in Hong Kong?