SmellScript.cs

void checkAllSmells()
{
    // Fetches all smell sources in the scene.
    SmellSource[] all_smells = FindObjectsOfType<SmellSource>();

    // We can multiple smell sources in our game
    foreach (SmellSource smell in all_smells)
    {
        // We calculate the intensity of the smell here
        float intensity = smell.getSmellIntensityAt(transform.position);

        // Add your logic here
    }
}