When choosing hi vis polo shirts for yourself or your workers it is worth considering a couple of things.
The type of material it is made from is often one of the first considerations. The most common is micromesh which is in basic terms polyester with small holes to help it breath. The big advantage of micromesh is that it is easy care, it drys quickly and is hard wearing. In the heat its advantage is that moisture runs off the fabric.
The disadvantage of micromesh is that it is a synthetic material, some people / workers can be more sensitive to this type of material, whilst rare people have reported rashes, allergies and that they are hot to work in.
Cotton hi vis polo shirts are made from a natural fiber and breath better than synthetic fibers. This is often considered more comfortable to work in, especially in high temperatures. The disadvantages however are that cotton hi vis is often not as bright as achieved on polyester, cotton also will shrink and the colour will fade with time.