Check your spelling. Use short sentences and simple grammar. Choose words that will be understood worldwide, not just in your own country or culture. Solicit comments on a draft before sending it out. Your message will circle the earth, so double-check.
Errors can be disastrous.
Do not overstate. Do not plead. Do not rant about the urgency of telling everyone in the world about your issue. And if the issue really is time-critical then just explain why, in sober language. Do not get obsessed with the immediate situation at hand.