From vessel to room
From liquid transport to exposure in the room
The mechanism needs no powered device, yet it continuously responds to the physical properties of the liquid, stick, and surrounding air.
How liquid enters and moves through a stick
The porous structure allows liquid to move upwards through capillary action.
The capillary beginning
When the lower end is immersed, liquid is absorbed by the porous structure and transported towards the exposed section through capillary action. The RIVM describes reed diffusers on this basis as continuous-release sources. [2]
In the Zhang study, behaviour in the exposed section was attributed mainly to internal diffusion through the synthetic stick, stick-to-air partitioning, and external convection. The study did not compare different stick materials or liquids with different viscosities, so its results apply to the specific system tested. [1]
The exposed section follows different physics
Zhang et al. (2024) studied a diffuser with synthetic-fibre sticks in a full-scale chamber. Across 120 hours, they concluded that capillary influence above the bottle opening was limited. Internal diffusion, interfacial partitioning, and convection into room air better explained release. [1]
This does not remove capillary action from the mechanism. It shows that transport out of the vessel and transfer into air are successive stages with different resistances. [1]
How fragrance molecules leave the stick
Each constituent partitions differently between liquid, surface, and air.
Not every material evaporates at the same rate
A fragrance mixture contains compounds that behave differently as they enter the air. In the short-term test, Zhang et al. determined different transport parameters for each compound. They did not follow a complete vessel through depletion. The paper presents faster depletion of some compounds only as a possibility for future research. [1]
In the Zhang study, linalyl acetate, linalool, and α-pinene were the three most abundant measured constituents. Concentrations were lower at 16°C than at 25°C, but the proportions of most VOCs changed very little. The result belongs to that study’s lavender liquid, not to every reed diffuser. [1]
More exposed area increases exchange
In the Zhang model, adding sticks increased the emission rate and equilibrium concentration. Separately, the RIVM describes flipping as exposing a wetted end directly to air and temporarily accelerating evaporation. Both practices can increase the consumption rate, but neither source directly measured perceived intensity. [1][2]
How position shapes the room experience
Once evaporation occurs, the room itself becomes part of the mechanism.
Temperature affects transport and concentration
In controlled tests at 16°C and 25°C, the lower temperature was associated with lower equilibrium concentrations and slower internal diffusion through the sticks. The effect varied by compound, while the relative composition of most VOCs changed very little. [1]
The result shows that room temperature can alter the release rate. The study compared two controlled temperatures and did not test direct sunlight or particular heat sources. [1]
Concentration is not uniform across a room
Computational simulations by Zhang et al. showed that source position and air-exchange rate materially changed volatile-compound distribution and calculated concentrations in the breathing zone. The study did not assess subjective fragrance perception. [1]
What formulation means for performance and exposure
Carrier and fragrance mixture both participate in evaporation.
Broad labels do not describe the complete formulation
The labels alcohol-based and alcohol-free describe one aspect of a formulation. They do not, on their own, show which constituents are released or at what concentrations. Overall behaviour depends on the particular liquid, fragrance mixture, sticks, and conditions of use. [1][4]
Likewise, the term VOC classifies an organic compound by its volatility. It is not a risk verdict. Assessment requires the identity of the compound, its concentration, duration, and an exposure scenario. [3][4]
Continuous release needs a realistic scenario
Karr et al. (2022) assessed 15 air-freshening products, including six passive diffusers. Their mean-use scenarios raised no concern for chronic exposure. Under some reasonable worst-case scenarios, chronic exposure could exceed selected health reference values, and one-hour acute exposure could also exceed selected standards. These results apply to the products and scenarios assessed. [3]
Volatile compounds can also react with other constituents of indoor air, including ozone. The extent of this secondary chemistry depends on the particular molecules, their concentrations, and room conditions. [4][5]