On 10 May 2025 a fire at the warehouse of a pool-maintenance products distributor in Vilanova i la Geltrú, Catalonia, Spain, led the authorities to activate the external emergency plan for the chemical sector and to order residents of five towns to stay indoors. The warehouse held around 70 tonnes of chlorine tablets. The fire started shortly after 02.00, the phone alert went out at 04.30, and three people were treated for minor injuries. The fire chief explained that every time crews applied water to stop the fire spreading, a chemical reaction fed the toxic cloud (Ara, 2025).
As of early June 2025 we had not found an official report on the cause, so we do not speculate about it. The event does show a pattern: materials that are stable while dry and segregated can become a source of toxic gas when they meet water or another substance. Calcium hypochlorite and chlorine tablets are also stored at hotels, pools, water treatment plants and distributors in Indonesia. This article explains one tool for reading that risk, the chemical reactivity matrix, with its steps, an example and its limits.
Water as Trigger
The US Environmental Protection Agency notes that oxidising pool chemicals, including calcium hypochlorite, sodium hypochlorite and chlorinated isocyanurates, can react when wetted by a small amount of water or mixed with incompatible materials, raising the temperature and releasing toxic gas. Documented water sources include roof leaks, wet floors, sprinkler leaks and wash-down water. Accidental mixing has occurred through shared tools, swept-up spills and waste containers (United States Environmental Protection Agency [US EPA], 2001).
The U.S. Chemical Safety and Hazard Investigation Board report on an event at a Louisiana plant on 27 August 2020 explains the mechanism. Trichloroisocyanuric acid (TCCA) wetted by a small amount of water can react, generate heat and decompose, releasing chlorine gas and possibly forming explosive nitrogen trichloride. Hurricane winds damaged the buildings, rainwater got in, and the decomposition caused a fire and a gas plume that led to a shelter-in-place order. It also lists at least three toxic releases between 1988 and 2019 linked to rainwater contacting TCCA (U.S. Chemical Safety and Hazard Investigation Board [CSB], 2023).
Water also plays a double role in firefighting. The product safety data sheet in the Louisiana case stated that a fire involving TCCA should be flooded with water, and the CSB found that a delay of about 5.5 hours in applying large volumes of water increased the amount of material that decomposed and the chlorine released (CSB, 2023). The EPA makes the same point: a little water triggers the reaction, while firefighting needs a copious flow applied by trained responders (US EPA, 2001).
Indonesian Legal Basis
Minister of Manpower Decree No. KEP.187/MEN/1999 on the control of hazardous chemicals in the workplace requires employers who use, store, produce or transport hazardous chemicals to control them (Article 2), including through safety data sheets and labels (Article 3). The safety data sheet must cover storage and handling as well as stability and reactivity (Article 4(1)). The decree also defines reactive and oxidising materials (Article 12), sets threshold quantities of 10 tonnes for oxidisers and 50 tonnes for reactive materials (Article 14), and classes companies above those thresholds as having major hazard potential (Article 15) (Departemen Tenaga Kerja [Depnaker], 1999).
Building the Matrix
A reactivity matrix lists every material in both the rows and the columns and marks each intersection as compatible, caution or incompatible. The Center for Chemical Process Safety offers the Chemical Reactivity Worksheet, free software that predicts the reactivity of mixtures of common chemicals and produces a compatibility chart (Center for Chemical Process Safety [CCPS], 2016). The steps are:
- List every material in one area, including cleaning agents, old stock, pallets and oils.
- Add water as its own row, since it can come from roofs, floors, sprinklers, hoses and cleaning.
- Take data from the storage and the stability and reactivity sections of each safety data sheet.
- Fill in every cell and flag incompatible pairs for physical separation.
- Test each incompatible pair against spill, wetting, fire and mixing-in-waste scenarios.
- Add up quantities by hazard category and compare them with the threshold quantities.
Hotel Store Example
Take a hotel engineering store holding granular calcium hypochlorite, TCCA tablets, hydrochloric acid for pH control, liquid sodium hypochlorite, floor cleaner, generator diesel and wooden pallets. With water added there are eight rows, so the matrix has 8 × 7 / 2 = 28 pairs to assess.
Based on the EPA guidance, the pairs likely to be flagged are the chlorine oxidisers with small amounts of water, with combustibles such as diesel and pallets, and with other pool chemicals, because mixing can release chlorine (US EPA, 2001). Follow-up actions can include racks that keep packages off the floor, separate rooms for acids and oxidisers, and dedicated scoops and spill containers for each material. The fire scenario is then agreed with the local fire service before anything happens.
Limits of the Method
A matrix is only as good as its inputs. Safety data sheets can be incomplete, software predictions are not laboratory tests, and slow humidity-driven reactions are easily missed (US EPA, 2001). We also note that the reactive-material criterion in KEP.187/MEN/1999 refers to reactions with water that release heat and flammable gas, whereas wetted TCCA releases toxic gas (Depnaker, 1999; CSB, 2023). In our view, gaps like this show why a regulatory class cannot replace a pair-by-pair assessment. The CSB report likewise warns that relying only on lists of chemicals is not enough to manage reactive hazards (CSB, 2023).
How Nusa Safety Helps
Nusa Safety helps companies review reactive chemical management through a Process Safety Management Review and HAZID studies, including building reactivity matrices for stores and process areas. Fire protection and emergency readiness can be checked through FESA and then practised through emergency preparedness drills. Nusa Safety issues internal training certificates only and is not a PJK3.
This article is general information and not legal or professional advice for any specific case.
References
- Ara. (2025, May 10). The lockdown has been lifted in Vilanova i la Geltrú, Roquetas del Garraf, Cubelles, Cunit and Calafell due to a warehouse fire. Ara in English. https://en.ara.cat/society/fire-in-chemical-plant-forces-vilanova-geltru-roquetes-cubelles-cunit-and-calafell-to-be-confined_1_5375010.html
- Center for Chemical Process Safety. (2016). Chemical Reactivity Worksheet (Version 4.0) [Computer software]. American Institute of Chemical Engineers. https://www.aiche.org/ccps/resources/chemical-reactivity-worksheet
- Departemen Tenaga Kerja. (1999). Keputusan Menteri Tenaga Kerja Nomor KEP.187/MEN/1999 tentang Pengendalian Bahan Kimia Berbahaya di Tempat Kerja [Minister of Manpower Decree No. KEP.187/MEN/1999 on the control of hazardous chemicals in the workplace]. https://jdih.kemnaker.go.id/peraturan/detail/2608/keputusan-menteri-nomor-187-tahun-1999
- U.S. Chemical Safety and Hazard Investigation Board. (2023). Trichloroisocyanuric acid reaction, decomposition, and toxic gas release at Bio-Lab, Inc. (Report No. 2020-05-I-LA). https://www.csb.gov/assets/1/6/BioLab_Investigation_Report_2023-4-24.pdf
- United States Environmental Protection Agency. (2001). Safe storage and handling of swimming pool chemicals (Chemical Safety Alert EPA 550-F-01-003). https://www.epa.gov/sites/default/files/2013-11/documents/spalert.pdf
