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Modeling of methyl isothiocyanate air concentrations associated with community illnesses following a metam-sodium
Michael O'Malley1, Terrell Barry, Marylou Verder-Carlos
1Employee Health Services, University of California, Davis, California 95616, USA. maomalley@ucdavis.edu
Background:
A series of illnesses occurred in Earlimart, CA on November 13, 1999 following a metam-sodium sprinkler application to a potato field on the southern edge of the town.
Methods:
Case information was obtained from direct interviews, from illness complaints filed with state or county agencies, or from illness reports filed by community physicians for 173 subjects. Air concentrations of the by-product methyl isothiocyanate (MITC) during the episode were estimated based upon the Industrial Source Complex Short-Term (ISC3) air dispersion model, estimates from prior metam-sodium monitoring studies, and data from weather stations in two adjoining communities.
Results:
Weather station data indicated that the November 13th incident corresponded with a temperature inversion at approximately 5:00 p.m. and a shift in the direction of prevailing wind from northwest to southeast. On the edge of Earlimart, 1-hr time weighted average (TWA) MITC concentration estimates ranged from 0.5 to 1 ppm. Of the 173 subjects, 170 had MITC-compatible symptoms, including eye or upper respiratory irritation (77.6%), non-specific systemic symptoms (64.7%), and lower respiratory symptoms (20.0%). 78.2% were exposed within 0.5 miles of the northern edge of the treated field where average 1 hr MITC concentrations exceeded the 800 ppb ocular irritation threshold.
Conclusions:
ISC3 modeling is a useful means of evaluating MITC concentrations for illness episodes when industrial hygiene assessment is not possible.
Insights
A 1999 California pesticide exposure incident linked metam-sodium (MITC) to widespread illness. Air modeling accurately estimated methyl isothiocyanate (MITC) levels, confirming exposure risks.
Area of Science:
- Environmental Health
- Toxicology
- Epidemiology
Background:
- Investigated a cluster of illnesses in Earlimart, California, on November 13, 1999.
- Illnesses followed a metam-sodium pesticide application near a residential area.
Purpose of the Study:
- To assess the relationship between metam-sodium exposure and reported illnesses.
- To evaluate the utility of air dispersion modeling for estimating chemical concentrations during an environmental health incident.
Main Methods:
- Collected case information via interviews and official complaints from 173 subjects.
- Estimated air concentrations of methyl isothiocyanate (MITC) using the ISC3 air dispersion model and meteorological data.
- Correlated estimated MITC levels with reported symptoms and proximity to the pesticide application site.
Main Results:
- A temperature inversion and wind shift coincided with the incident.
- Estimated 1-hour average MITC concentrations near the field ranged from 0.5 to 1 ppm.
- 170 out of 173 subjects reported MITC-compatible symptoms, with 78.2% exposed within 0.5 miles of the treated field.
Conclusions:
- The Industrial Source Complex Short-Term (ISC3) model effectively estimated MITC concentrations.
- Air dispersion modeling is a valuable tool for assessing chemical exposures in public health investigations when direct monitoring is not feasible.
