A Near-Explicit Reaction Mechanism of Chlorine-Initiated Limonene: Implications for Health Risks Associated with the Concurrent Use of Cleaning Agents and Disinfectants

Limonene, a key volatile chemical product (VCP) commonly
found
in personal care and cleaning agents, is emerging as a major indoor
air pollutant. Recently, elevated levels of reactive chlorine species
during bleach cleaning and disinfection have been reported to increase
indoor oxidative capacity. However, incomplete knowledge of the indoor
transformation of limonene, especially the missing chlorine chemistry,
poses a barrier to evaluating the environmental implications associated
with the concurrent use of cleaning agents and disinfectants. Here,
we investigated the reaction mechanisms of chlorinated limonene peroxy
radicals (Cl-lim-RO2•), key intermediates in determining
the chlorine chemistry of limonene, and toxicity of transformation
products (TPs) using quantum chemical calculations and toxicology
modeling. The results indicate that Cl-lim-RO2•
undergoes a concerted autoxidation process modulated by RO2• and alkoxy radicals (RO•), particularly emphasizing
the importance of RO• isomerization. Following this generalized
autoxidation mechanism, Cl-lim-RO2• can produce
low-volatility precursors of secondary organic aerosols. Toxicological
findings further indicate that the majority of TPs exhibit increased
respiratory toxicity, mutagenicity, and eye/skin irritation compared
to limonene, presenting an occupational hazard for indoor occupants.
The proposed near-explicit reaction mechanism of chlorine-initiated
limonene significantly enhances our current understanding of both
RO2• and RO• chemistry while also highlighting
the health risks associated with the concurrent use of cleaning agents
and disinfectants.