Missing Field Exception

I am getting this:

MissingFieldException: Field ‘HealthDisplay.emit’ not found.
Boo.Lang.Runtime.DynamicDispatching.PropertyDispatcherFactory.FindExtension (IEnumerable`1 candidates)
Boo.Lang.Runtime.DynamicDispatching.PropertyDispatcherFactory.Create (SetOrGet gos)
Boo.Lang.Runtime.DynamicDispatching.PropertyDispatcherFactory.CreateSetter ()
Boo.Lang.Runtime.RuntimeServices.DoCreatePropSetDispatcher (System.Object target, System.Type type, System.String name, System.Object value)
Boo.Lang.Runtime.RuntimeServices.CreatePropSetDispatcher (System.Object target, System.String name, System.Object value)
Boo.Lang.Runtime.RuntimeServices+c__AnonStorey16.<>m__C ()

What I’m trying to do is make my third person controller start to smoke (particle emmiter) once I dip below a certain value (in the health display).

Any ideas? Your help is greatly appreciated!

Ahh sorry! Total rookie at this, first time using Javascript... This is what I have coded:

var HealthSmoke: ParticleEmitter;

function Update () {
    if (HealthDisplay.health <=20){
       tag == "healthSmoke";
       HealthSmoke.emit = true;
       return "was false, stopped at condition";
    }

    else {
       HealthSmoke.emit = false;
    }
}

I'm not sure if using both the true/false statement and the return statement is redundant? What I'm trying to do is have a smoke trail follow my little car once the health dips below 20%... Thanks!