function fade_dust(PARTICLE *p)
{
p.alpha -= 10 * time_step;
p.size += 3 * time_step;
if (p.alpha < 0) { p.lifespan = 0;}
}

function dust(PARTICLE *p)
{
//vec_randomize(vecDust, 5);
p->vel_x = 1 - random(1);
p->vel_y = 1 - random(1);
p->vel_z = 0 + random(2);
p.alpha = 50 + random(5);
p.bmap = sprite_fog;
p.size = 7;
//p.lifespan = 50;
p.flags |= ( MOVE);
p.event = fade_dust;
}

function particle_start(){
while(1){
vec_set (vecDust.x, vector (20,0,-1));
vec_rotate (vecDust.x, my.pan);
vec_add (vecDust.x, my.x);
effect (dust, 1, vecDust, nullvector);
wait(-0.50);}
}

action fly()
{
particle_start();
...