Large venous air embolisms (centimeters) may lead to death if they become lodged in the heart. if big enough they can cause the heart to loose prime and as a result stop the bood from flowing from the right ventricle to the lungs.
Gas embolism into an artery, termed arterial gas embolism, or AGE, is a more serious matter than in a vein, since a gas bubble in an artery may directly cause stoppage of blood flow to an area fed by the artery. The symptoms of AGE depend on the area of blood flow, and may be those of stroke or heart attack if the brain or heart, respectively, are affected. The amount of arterial gas embolism which causes symptoms depends on location, but in the brain may be a bubble with a volume only a fraction of a milliliters.
(sourced from wikipedia)