Generally there are possibly 2 types of causes for Asthma problem, internally or externally. Crystal Salt only release long negative ion which in turn clean the air around you. If your issues are mainly triggered by dirty air, then Crystal Salt can help you. But if it is your own body function that cause the issue ie. too weak, too sensitive, hormone etc. then Crystal Salt can help less.
While the negative ion is scientific and will help everyone the same way, another property of the lamp is Color Therapy. That on the other hand will only work for about 60% of the people in statistic. This is due to wrong initial diagnostic and dynamic of human nature.
That is why you will hear some works magically for some people while some really curse on it. In short and summary, it may or may not work on you personally. Only after getting one then you will know.
On the color therapy part, changing different lamps may help until you find one that suits you. But on the ionizing or air cleaning part, even the worse salt lamp is about 96% pure ( content of salt in it ) so almost all lamps will work almost in similar power.