Respiratory and nervous system effects of a hydrogen sulfide crisis in Carson, California

In October 2021, many residents in Carson, California experienced malodors, headaches, and respiratory symptoms. Hydrogen sulfide (H₂S), a toxic odorous gas, was measured in Carson at concentrations up to 7000 parts per billion (ppb) and remained above California's acute air quality standard of 30 ppb for about a month. Research on how low- and medium-level H₂S exposure affects the respiratory and nervous systems has yielded conflicting results, and few studies have examined the effects of subacute H₂S exposure.

We calculated daily rates of emergency department (ED) visits with various respiratory and nervous systems diagnosis codes in Carson area ZIP codes (≤6 km from event's epicenter) and in Los Angeles County ZIP codes >15 km from event's epicenter (control area). Using controlled interrupted time series, we compared ED visit rates during the month of the H₂S crisis in Carson to the predicted rates had the incident not occurred, based on 2018-2021 ED trends, and controlling for ED visit rate changes in the control area.

We observed a 24 % increase in ED visit rate for all respiratory system diseases (rate ratio = 1.24, 95 % CI: 1.16, 1.32), a 38 % increase for asthma (RR = 1.38, 95 % CI: 1.26, 1.50), a 26 % increase for acute upper respiratory infections (RR = 1.26, 95 % CI: 1.13, 1.38), a 21 % increase for dizziness (RR = 1.21, 95 % CI: 1.04, 1.38), and a 25 % increase for migraines and headaches (RR = 1.25, 95 % CI: 1.13, 1.36) in the Carson area during the first month of the H₂S event compared to the expected rates.

This H₂S crisis was associated with increased ED visit rates for multiple respiratory and nervous system outcomes. Reducing H₂S exposure and improving to response during H₂S episodes may improve public health.