Indoor Air Pollution: Aerosol Propellants Study

Indoor air frequently contains higher concentrations of pollutants than outdoor air, with aerosol propellants identified as a primary contributor. A study published in Environmental Science: Processes & Impacts (2021) analyzed 60 UK homes and found that n-butane and propane—common propellants in sprays—were the most abundant volatile organic compounds (VOCs) indoors. The research, conducted by the University of York and Givaudan, suggests that reducing hydrocarbon propellant use could significantly lower domestic emissions.

Hydrocarbon Propellants Dominated Indoor Air in 60-Home Study

Researchers from the Wolfson Atmospheric Chemistry Laboratories and Givaudan collected 360 air samples over 72-hour periods from 60 UK homes across two seasons. They measured 40 VOCs while participants logged product usage, including air fresheners, cleaners, and personal care items. Results revealed indoor air heavily influenced by aerosol products.

Total indoor VOC concentrations varied widely, ranging from 16.6 to 8150 μg/m³ in winter. N-butane, a common propellant, was the most prevalent compound, with levels between 1.5 and 4630 μg/m³. Propane and ethanol also contributed significantly. Indoor VOC levels consistently exceeded outdoor concentrations in all winter samples and 84% of summer samples, confirming indoor sources as the primary emitter.

Ally Lewis of the National Centre for Atmospheric Science noted that while overall product-use data weakly predicted air quality, specific propellant VOCs like n-butane directly correlated with aerosol usage, making them a priority for emission reduction.

Regulatory and Formulation Challenges of Propellants

Hydrocarbon propellants such as n-butane, isobutane, and propane are cost-effective but contribute substantially to a product’s VOC content under regulations like California Air Resources Board (CARB) standards. CARB limits VOCs in aerosol air fresheners to 25% by weight for most categories, with some restricted to 10%.

Since propellants, solvents, and volatile fragrance components all count toward VOC limits, hydrocarbon propellants can consume much of the allowable budget, forcing compromises in fragrance strength. The study’s findings linking propellants to indoor pollution add consumer health concerns to regulatory constraints.

Alternative Propellant Systems and Compliance Strategies

Formulators can adopt several approaches to meet VOC limits:

• Exempt propellants: Compressed gases (N₂, CO₂, N₂O) avoid VOC contributions but may require specialized packaging and alter spray characteristics.
• Hydrofluorocarbons (HFCs): Compounds like HFC-152a are exempt but face phase-downs due to global warming potential.
• Non-aerosol systems: Pump sprays, reed diffusers, or solid formats eliminate propellant VOCs entirely, freeing the VOC budget for fragrance.

Practical Steps for Formulators

To optimize compliance and air quality:

1. Calculate total VOC content: Include propellant, solvents, and volatile fragrance components as a percentage of product mass.
2. Evaluate propellant alternatives: Assess exempt options for spray performance, cost, and compatibility.
3. Adjust solvent systems: Reduce ethanol or increase water content where possible.
4. Optimize fragrance: Collaborate with suppliers to maximize non-volatile or exempt ingredients without sacrificing scent performance.

The study underscores that aerosol propellants significantly degrade indoor air quality. For manufacturers, balancing regulatory compliance, product performance, and consumer health requires meticulous propellant selection and formulation adjustments.

Sources:
University of York & Givaudan. (2021). Environmental Science: Processes & Impacts