Baffling Brainteasers

Which chemical causes the burning taste sensation when eating chilies?
What is the name of the layer of air closest to us in the atmosphere?
Which is the rarest blood type in humans?
What does the Scoville Heat Unit Scale measure?

Email your answers to:  paula@caltagmedsystems.co.uk.
All correct entries will be entered into a draw to win a £50/€50 Amazon Voucher.