There are many levels of compression that compression stockings come in and they are all for different purposes. Some work better than other depending on the situation. Just because 30-40 mmHg is a higher level of compression than a 15-20 mmHg doesn't mean that it's more effective for you.
The 15-20 mmHg compression is a low level of compression and is considered over the counter. They are perfect for air travel, long car rides and supporting your legs if you stand or sit for long periods of time. Many people find that wearing a lower level of compression is great as a daily sock. You don't need to have a problem with your veins or legs to wear compression stockings. I wear them and a lot of the staff at Leg Smart wear them everyday as well.
The 20-30 mmHg, 30-40 mmHg and 40-50 mmHg levels of compression are stronger and have more sizing options. You'll also notice that they come in more style options such as knee high or thigh high. The higher level of compression also limits the kinds of materials that are used to make the sock or stockings. These materials are usually advanced synthetics to help regulate heat and moisture. In addition, they also help the compression stocking be more stretchy. So you don't find a lot of compression socks with a high cotton content in a 40-50 mmHg thigh high. If you're not sure what level of compression to choose, always consult your doctor or medical professional.