You are a PA working in A&E and you are assessing someone with an acute exacerbation of asthma. You need to grade the exacerbation prior to sending this patient to be reviewed by the respiratory team. His observations are as follows: RR 25, HR 110, O2 saturations 92%. He is also using his accessory muscles and his PEFR is 33-50% of predicted. What severity of exacerbation is this?