The U.S. Environmental Protection Agency says the Muscatine area doesn't meet air quality standards for sulfur dioxide.
The Iowa Department of Natural Resources said this week that the EPA has determined that Muscatine and parts of eastern Muscatine County didn't meet standards after assessing air monitoring data from 2009 through 2011.
The EPA's decision means the state must submit a plan to demonstrate how to meet sulfur dioxide standards within five years.
The EPA notes a number of potential sources for the high sulfur dioxide level, including a power plant in Louisa County.
High sulfur dioxide levels can cause difficulty breathing and worsen asthma symptoms. Children, the elderly, and people with lung disease are most susceptible.