Siberian Fir, Silver Fir and Engelman Spruce are three that generally have at least 25% d-limonene.
Why do you care about d-limonene? Well maybe you don't yet, but check this out . . .
The immune stimulating effects of d-limonene have been well researched, and the component is known to stimulate white blood cells that help fight infection. This component also offers anti-inflammatory, analgesic, and antioxidant effects. Seems worth a little attention . . . right?
The group of oils with the highest d-limonene content are the citrus oils. Orange and Grapefruit contain about 95% d-limonene! Orange, Grapefruit, Siberian Fir and Engelman Spruce create a beautiful aromatic blend that can be very helpful when you feel like you're coming down with a cold, have a sinus infection, or headaches from congested sinuses. The Siberian Fir brings a calming aspect to the blend with its ester content. If you want a deeper, lower note, try adding a few drops of Palo Santo, which also happens to have a very high percentage of d-limonene (almost 70%).
Inhaler for relieving a cold, sinus infection or headache from congested sinuses:
Orange -- 3 drops
Grapefruit -- 3 drops
Siberian Fir -- 3 drops
Engelman Spruce -- 4 drops
Palo Santo -- 2 drops
Add the essential oils to a blank inhaler. This blend can also be diluted in jojoba and massaged into the back of your neck.