What does paint carry besides colour? The chemistry of water-based formulations and the air we breathe

When we decide to refresh the walls of a room, we usually focus on the colour palette and the aesthetic finish. However, the air we live with after painting depends directly on the product’s technical composition.

In the decorative coatings industry, replacing organic solvents with water-based systems marked a historic step forward. Nevertheless, water-based formulations are not simply “water and colour”: to keep a liquid stable on a shelf for months, prevent uncontrolled foaming and create a uniform film as it dries, industrial chemistry must rely on a complex combination of auxiliary additives.

International organisations dedicated to respiratory health and indoor air quality—such as the EPA (US Environmental Protection Agency), the AAFA (Asthma and Allergy Foundation of America) and ECHA (European Chemicals Agency)—highlight the importance of closely examining emission sources in enclosed spaces, where we spend around 90% of our time on average.

Which additives are needed to keep a water-based mixture stable, and how do they interact with the indoor environment?

1. In-can preservatives and biocides: microbiological protection

Stagnant water at room temperature provides a favourable environment for the growth of bacteria, fungi and microorganisms. Without added preservatives, stored liquid paint would deteriorate, producing unpleasant odours, a loss of viscosity and gas formation.

To ensure stability during storage (in-can preservation), traditional water-based formulations rely on synthetic biocides, particularly the isothiazolinone family:

• Methylisothiazolinone (MI), benzisothiazolinone (BIT) and methylchloroisothiazolinone (MCI): Analytical studies of commercial water-based paints have detected MI and BIT in a large majority of samples, although the industry is working hard to reduce their content further.
• Sensitivity and contact allergies: The AAFA and occupational dermatology networks such as the IVDK highlight that isothiazolinones are significant skin sensitisers. Continued exposure or airborne contact with these additives can trigger reactions in people with allergic contact dermatitis or airway hyperreactivity.

From a regulatory perspective, the EU Biocidal Products Regulation (BPR, Regulation (EU) No 528/2012) imposes increasingly stringent restrictions and labelling thresholds because of the allergenic and sensitising potential of these compounds.

2. Coalescing agents and co-solvents: film formation

In water-based paint, synthetic resin particles (the binder) are suspended in water. For these particles to join together and form a continuous, smooth and durable film as the water evaporates, a process known as coalescence is required.

Under varying temperature conditions, water alone does not always ensure that this process occurs optimally. Traditional formulations therefore incorporate auxiliary additives:

• Organic coalescing agents and glycol ethers: Slowly evaporating solvents that temporarily soften the resin to help the particles fuse together. As the paint dries on the wall, these compounds continue to be released gradually into the surrounding air.

• Volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs): Leading health bodies such as the WHO (World Health Organization) and the EPA warn that VOC emissions in enclosed spaces can trigger mucous membrane irritation, headaches or worsening asthma symptoms.

3. Defoamers and process-control additives

The industrial process of dispersing pigments at high speed in a liquid medium, as well as subsequent application with a roller or brush, encourages air to become incorporated into the mixture. If bubbles remain trapped, they cause surface defects (cratering or porosity) as the paint dries.

To counteract this natural tendency of water combined with the surfactants in the formulation, defoaming and deaerating agents need to be added:

• Compounds based on mineral oils, silicone emulsions or modified polymers.
• Their main function is to counteract a mechanical effect inherent in the agitation of liquids, ensuring a uniform finish.

4. Rheology modifiers and anti-settling agents

Mineral pigments and fillers (such as titanium dioxide or calcium carbonate) have a considerably higher density than water. During prolonged storage, gravity would cause these solids to settle at the bottom, forming a compact sediment that is difficult to redisperse.

To prevent this phase separation during commercial storage, liquid paints contain synthetic thickeners and rheology modifiers (cellulosic or associative acrylic types) that impart structural viscosity to keep the solid particles suspended.

Indoor Air Quality (IAQ) and Respiratory Health

Concern about what we breathe in our homes and workplaces has driven the development of international certification programmes specifically focused on healthy buildings:

• Indoor Air Comfort® GOLD (Eurofins): Sets Europe’s strictest limits for VOC and SVOC emissions from building materials 28 days after application.
• Asthma & Allergy Friendly® (Allergy Standards Ltd. / AAFA): Assesses decorating materials to ensure that they do not release irritants or allergens that could adversely affect people with asthma or respiratory sensitivities.

Understanding the function of each auxiliary ingredient enables more informed decisions, prioritising formulations that minimise unnecessary additives and protect indoor environmental comfort.

Eliminating the unnecessary to protect the air we breathe

Analysing paint chemistry is not about questioning the industry’s progress, but about asking how we can continue to improve the relationship between building materials, human health and the environment.

Reducing reliance on in-can biocides, volatile coalescing agents and synthetic process-control additives is the way towards indoor spaces with cleaner, safer air for everyone.

At ACTIVATE, we believe that the future of architecture and decoration lies in questioning every component from the outset, designing solutions that focus on the wellbeing of people and the planet.

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Sources and Technical References

1. US EPA (US Environmental Protection Agency): Indoor Air Quality (IAQ) and Volatile Organic Compounds (VOCs) in Building Materials. Guidance on environmental health and the impact of emissions in enclosed spaces.
2. Asthma and Allergy Foundation of America (AAFA) & Allergy Standards Ltd. (ASL): Asthma & Allergy Friendly® Certification Standards for Indoor Coatings. Criteria for assessing allergens, biocides and volatile compounds for people with asthma and sensitivities.
3. Eurofins Environment Testing / Indoor Air Comfort GOLD®: Criteria and regulatory limits for VOC, SVOC and formaldehyde emissions at 28 days (ISO 16000 and EN 16516 standards).
4. Lundov MD et al. (2014) & IVDK (2021): Emission of Isothiazolinones from Water-Based Paints (Environ Sci Technol) & Occupational contact dermatitis in painters. Epidemiological analyses of skin and respiratory sensitisation to preservatives in decorating products.
5. Regulation (EU) No 528/2012 (BPR): European Union Biocidal Products Regulation. Restrictions on use, risk assessment and labelling requirements for isothiazolinones (MI, BIT, MCI).