not sure of high altitude
but my wife has been making her own repellant,
and has been working fine for me, even in swampy skeeter infested lands
dont know exact mix
but something like
base: distilled water and witch hazel
oils: citronella, clove and something else i cant recall,
i like it as its natural and not loaded with deet and such
after using hers i wont use store bought repellant anymore