Aromatic diffuser products differ from candles in that fragrance release is not driven by heat combustion but by evaporation, capillary action, or ultrasonic atomisation. Each diffusion mechanism creates different formulation requirements, particularly around carrier choice, viscosity, and fragrance oil compatibility.

Reed diffuser with aromatic reed sticks in a glass vessel

Reed Diffusers

Reed diffusers are the most common ambient fragrance format in the European craft market. A liquid blend is placed in an open vessel with rattan or fibre reeds that draw the liquid upward by capillary action, allowing it to evaporate at the exposed tip.

Carrier Selection

The carrier in a reed diffuser blend determines evaporation rate, viscosity, and how well fragrance oils remain in solution. The most commonly used carriers among European craft producers are:

  • DPG (Dipropylene Glycol): A synthetic carrier with low evaporation rate, good solubility for most fragrance oils, and low skin sensitisation risk. Widely available and cost-effective. Mixes well with both synthetic fragrance oils and most essential oils.
  • IPM (Isopropyl Myristate): An ester carrier with moderate evaporation rate and a slightly thicker consistency. Noted for good fragrance throw and clean burn characteristics. More expensive than DPG.
  • Ethanol (denatured): A fast-evaporating carrier that can extend fragrance throw but reduces longevity significantly. Commonly used as a secondary carrier at 10–20% of total volume to improve initial scent impression.

Fragrance Concentration in Reed Blends

Reed diffuser blends typically contain 20–30% fragrance oil by volume, with the remainder made up of carrier. Higher concentrations above 30% are possible but may cause crystallisation or separation with certain fragrance materials, particularly those with high eugenol or vanillin content.

Rattan reeds draw liquid at different rates depending on reed diameter, porosity, and age. Reeds should be flipped weekly during the first two weeks to re-saturate the exposed tips. After approximately four to six weeks, reeds may become saturated with fragrance residue and should be replaced to maintain diffusion rate.

Reed Material

Rattan reeds are the traditional and most widely used material. Synthetic fibre reeds offer more consistent diffusion rate but are less biodegradable. Some producers working with purely natural product lines use dried grasses or wood sticks, though these have lower capillary performance than rattan.

Ultrasonic Diffusers

Ultrasonic diffusers use a small ceramic disc vibrating at ultrasonic frequency to break water-fragrance mixtures into fine mist particles. Unlike heat-based nebulisers, they do not warm the oil and are generally considered less likely to degrade heat-sensitive aromatic compounds.

Formulation for Ultrasonic Use

Ultrasonic diffusers are designed for water-based use. A standard approach is to add a few drops of pure essential oil or a water-soluble fragrance blend to the water reservoir. Oil-based fragrance oils are not compatible with ultrasonic diffusers unless they are specifically formulated as water-soluble concentrates, as standard fragrance oil will sit on the water surface and can clog the ceramic disc.

Essential Oil Compatibility

Essential oils for ultrasonic use do not require a carrier in the same sense as reed diffuser blends. However, certain essential oils — particularly those high in resinous compounds such as benzyl benzoate or methyl eugenol — can leave residue in the device over time. Regular cleaning cycles are standard practice with citrus or resinous oil blends.

Natural carrier oil used in aromatic formulations

Passive and Evaporative Diffusion

Passive diffusion covers a range of formats including scented ceramic plates, porous stones, fibre pads, and paper-based diffuser cards. In all cases, fragrance or essential oil is applied directly to an absorbent substrate and allowed to evaporate at room temperature with no mechanical assistance.

Formulation Considerations

Passive diffusers have the fastest initial release and shortest longevity of the three main format types. They are most effective with top-note-dominant essential oil blends, as middle and base notes require more time to volatilise and may not fully express in short-duration passive formats. A common application is aromatic sachets or drawer liners using dried botanicals combined with a fixative-forward essential oil blend.

European Regulatory Context for Diffuser Products

The regulatory classification of a diffuser product in the EU depends on its intended use claim. Products marketed solely as ambient fragrance fall outside the EU Cosmetics Regulation (EC 1223/2009). However, products making any claim related to the skin, health, or personal care may be drawn into cosmetic or even medicinal product classification, each of which carries distinct compliance obligations.

REACH and Fragrance Ingredients

The EU REACH Regulation (EC 1907/2006) applies to chemical substances including fragrance ingredients. Fragrance materials used in diffuser blends must appear on a Safety Data Sheet (SDS) if the product is sold to professional users. Consumer products are subject to labelling requirements under the CLP Regulation where classified substances are present above threshold concentrations.

IFRA Standards

The International Fragrance Association (IFRA) publishes usage guidelines for fragrance materials based on Quantitative Risk Assessment (QRA) methodology. For diffuser products, IFRA Category 10B (leave-on, rinse-off products not applied to skin) or Category 11A (non-skin-contact products) is typically applicable, depending on the specific format. Fragrance oil distributors supplying the European market generally provide IFRA compliance certificates with each batch.

Scent Profile Construction

Aromatic compositions for diffusers are typically structured around the classic top-middle-base note framework:

  • Top notes: High volatility, first impression. Examples include citrus oils (bergamot, lemon), light herbal notes (basil, eucalyptus). Evaporate within 30–60 minutes in passive formats.
  • Middle notes (heart notes): Medium volatility, main body of the scent. Examples include lavender, geranium, black pepper, clary sage.
  • Base notes: Low volatility, longevity and depth. Examples include cedarwood, vetiver, benzoin, labdanum. These anchor the blend and extend diffusion time in reed and passive formats.

When building a fragrance blend for reed diffuser use, a typical structural ratio might allocate 20–30% to top notes, 40–50% to middle notes, and 25–35% to base notes. These are starting proportions only — the final balance depends on the volatility characteristics of specific materials and the desired diffusion profile.

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