Why won't anything happen when my IntValue drops below zero?

Hey guys, so I've got this script that I've been working on for a while now and I'm very close to done with it, but there's one issue, it won't take damage or print when the value drops below zero. Here is the script,

local Player = game:GetService("Players").LocalPlayer
local Character = Player.Character or Player.CharacterAdded:Wait()
local Humanoid = Player.Character:WaitForChild("Humanoid")
local air = script.Parent
local gui = script.Parent.Parent.Parent.PlayerGui

if Character:FindFirstChild("SmallAir") then -- if u have small tank then air start is 30
    air.Value = 40
end


wait(8)
while wait(1) do -- diminish air every second/do damage at 0
    if Character:FindFirstChild("SmallAir") then
        air.Value = air.Value - 1
        print("Run Successfully")
            if air.Value >= 0 then
                print(air.Value.." air remaining.")
                gui.AirCanStat.HeadText.Label.Text = air.Value.."/40"
            if air.Value == 40 then
                gui.AirCanStat.BlueBody:TweenPosition(UDim2.new(0.023, 0,0.641, 0), 'Out', 'Linear', 1, false)
                gui.AirCanStat.BlueBody.BackgroundColor = BrickColor.new("Cyan")
            end
            if air.Value == 30 then
                gui.AirCanStat.BlueBody:TweenPosition(UDim2.new(0.023, 0,0.727, 0), 'Out', 'Linear', 1, false)
                gui.AirCanStat.BlueBody.BackgroundColor = BrickColor.new("Cyan")
            end
            if air.Value == 20 then
                gui.AirCanStat.BlueBody:TweenPosition(UDim2.new(0.023, 0,0.811, 0), 'Out', 'Linear', 1, false)
                gui.AirCanStat.BlueBody.BackgroundColor = BrickColor.new("Cyan")
            end
            if air.Value == 10 then
                gui.AirCanStat.BlueBody:TweenPosition(UDim2.new(0.023, 0,0.892, 0), 'Out', 'Linear', 1, false)
                gui.AirCanStat.BlueBody.BackgroundColor = BrickColor.new("Really red")
            end
            if air.Value == 0 then
                gui.AirCanStat.BlueBody:TweenPosition(UDim2.new(0.023, 0,0.988, 0), 'Out', 'Linear', 1, false)
            end
            if air.Value <= 0 then
                print("No air remaining!")
                Humanoid:TakeDamage(10)
            end
        end
    end
end

With this script, it works one time (when the value drops to exactly zero) but if I remove the equal sign and make it just if air.Value < 0 then but when I do this, nothing happens, no damage taken and no print, there is also no error, which is why I'm confused

Thanks!