Lemon grass ( and lemon thyme) contain citronella oil. Citronella oil repels mosquito. Plants release significant amounts of their repellent oils only when their leaves are crushed. I read the best way to use the citronella oil containing plants would be to rub crushed leaves on your skin. Be sure to "test" yourself for any allergy to these leaves by repeatedly rubbing a small amount of material on your inner forearm for a day or so. If there is no irritating skin reaction, it is safe to use the plants.
Hope it helps...