The lowest seat height seems to be about 50cm. It is a big chair for big people and I love it. It fully supports my back and is all in all comfy. But it is quite a wide chair.
The backrest reaches slightly above the middle of the head (my height is 184 cm), but there is no actual separately adjustable headrest.
This means that the head is not supported when working actively.
When watching TV, etc., in a relaxed position with the head leaning back, the head is supported.