✓ Accepted answer
Your problem is that it's only checking the health once, right when the first player loads into the game. To fix this, you could connect it to any one of a few possible events, or just use a loop. So, let's go ahead and use the HealthChanged event.
local Humanoid = script.Parent
Humanoid.HealthChanged:connect(function(NewHealth)
if NewHealth < 25 then
script.Parent.Parent.Torso.ParticleEmmiter.Enabled = true
else
script.Parent.Parent.Torso.ParticleEmmiter.Enabled = false
end
end)
All this does, is fires a function every time the Humanoids health changes. So it should, in theory track every health change that's made to the Humanoid.
So now your code should work properly. If you have any further problems/questions, please leave a comment below, and I'll see what I can do. Hope I helped :P