What Happened?

An over-pressurized chemical waste bottle exploded in a lab. The explosion was loud enough to be heard throughout the floor and on the floor below, but fortunately, the lab was unoccupied at the time. Several people entered the lab to assess the situation and observed shards of glass and corrosive liquid scattered throughout. A broken bottle cap was found several feet from the site of explosion. An acrid odor filled the hallway near the explosion site. The lab members evacuated the lab and reported the incident to the EH&S on-call specialist, notified the EH&S on‑call specialist, who in turn coordinated response efforts with the fire department and the hazardous waste team. Due to the extent and unknown nature of the chemical contamination, the clean-up was carried out by an outside contractor. A hazardous waste label recovered at the scene indicated tha the bottle had contained a mixture of 50% nitric acid, 25% sulfuric acid, and 25% hydrochloric acid, similar to aqua regia. 

What Was The Cause?

Immediate cause: The immediate cause was the explosion of an over‑pressurized chemical waste bottle containing a mixed strong‑acid solution, which led to the sudden release of energy, glass fragments, and corrosive liquid into the lab.

Underlying causes:

Aqua regia (3 parts of concentrated hydrochloric acid and 1 part of concentrated nitric acid) and similar strong acids are used for removing trace amounts of organic compounds and metals from glassware. After use, it is often wrongly stored in closed containers and sometimes even re-used. Some plausible underlying causes could include:

• Not recognizing that nitric acide is not only a strong acid but also a potent oxidant. Thus, contamination of nitric acid solutions with organic compounds that act as reducing agents can cause a violent exothermic reaction. The following could have resulted in contamination:
  • The waste container was not properly cleaned before adding aqua regia 
  • Aqua regia was heavily contaminated with organic materials during cleaning activities 
  • Organic waste was improperly added to the waste container
• Aqua regia decomposes over time, generating toxic nitrosyl chloride, chlorine gas, and nitrogen dioxide. If sulfuric acid is present, toxic sulfur dioxide is also released. Failure to recognize or communicate that this waste mixture can generate gas or heat and build internal pressure over time and storing aqua regia–like mixtures in a sealed bottle).

Root causes:

1. Inadequate hazardous waste program guidance for reactive acid mixtures. Institutional waste procedures did not clearly prohibit or flag pressure‑generating acid mixtures (e.g., aqua‑regia–type wastes) in tightly sealed containers, or this guidance was not effectively implemented.
2. Insufficient training and competency verification on waste compatibility and reactivity. Personnel who generated or handled the waste were not adequately trained (or periodically refreshed) on incompatibilities and pressure‑buildup hazards associated with mixed strong acids and other reactive components.
3. Weak controls over lab‑level waste practices and supervision. There was no effective system to review, approve, or spot‑check lab‑specific waste practices (e.g., an SOP specific for aqua regia and similar solutions detailing what is going into mixed acid waste, whether vented caps or alternative collection methods are used), indicating a gap in supervisory oversight and EH&S auditing.
4. Deficient risk assessment process for high‑hazard waste streams. The organization lacked a formal process to identify “high‑hazard” waste streams (such as oxidizing acid mixtures) and apply enhanced controls (dedicated containers, vented caps, volume limits, time limits, or special collection procedures).
5. Communication and labeling gaps about special handling requirements. Even though a label was present, it did not trigger special handling or storage controls for this particular mixture, suggesting that the labeling/communication system was not designed to reliably convey reactivity and pressure hazards for waste, only composition.

How Can Incidents Like This Be Prevented?

• Work with aqua regia or similar solutions only inside the chemical fume hood wearing appropriate PPE. 
• Dispose of aqua regia and other nitric acid solutions in clean glass containers with vented caps. Prohibit or tightly control aqua‑regia–type and other pressure‑generating acid mixtures in sealed waste bottles. 
• Limited storage times of aqua‑regia or similar solutions.
• Segregate incompatible wastes and identify high‑hazard waste streams in written SOPs (e.g., separate strong oxidizing acids and reactive mixtures, never add organic waste to nitric acid solutions).
• Require complete, standardized waste labels that capture both composition and key hazards (e.g., “strong oxidizing acid mixture – pressure/heat generation risk”) to trigger special handling and storage.
• Provide lab‑specific training on waste compatibility, reactivity, and case studies of waste‑bottle explosions, with explicit examples of “never mix/never seal” combinations.
• Ensure PIs/supervisors and lab managers actively review waste practices during inspections and onboarding, and reinforce expectations through periodic refreshers.
• If possible, substitute aqua regia cleaning solutions with safer alternatives, including ultrasonic baths, Alconox or similar detergents, Pierce RBS-35 (available from VWR), or similar detergents or biodegradable surfactants.

 Resources

• Aqua regia
• Aqua regia podcast
• Vented cap video
• Examples of different styles of vented caps