The substance is not completely airtight (I like that), there is a vent at the upper back and the armholes are well dimensioned. Not too big and not too small. At the edge there is a rubber band. The color and reflectors serve their purpose very well and you will be seen.
I can confirm, however, that the new vest considerably stinks. Whether that is harmful, I can not judge, but one or two that will interfere.
Nevertheless, for me fits the function with a great look that makes together with the extremely reasonable price of about 23 EUR a full five stars.