Write SQL query to solve given problem: What percentage of patients born in 'Pembroke MA US' have 'allergy to grass pollen'?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(SUM(CASE WHEN T2.DESCRIPTION = 'Allergy to grass pollen' THEN 1 ELSE 0 END) AS REL) * 100 / COUNT(T1.patient) FROM patients AS T1 INNER JOIN allergies AS T2 ON T1.patient = T2.PATIENT WHERE T1.birthplace = 'Pembroke MA US'