It is not well known, or at least not well publicized, that fireworks — from sparklers to professional displays — leave behind a fair amount of waste, while releasing noxious gases and heavy metals. The large displays emit a fog of chemicals and toxins with various levels of health risks. Perchlorate is a widely used rocket propellant with a history of finding its way into groundwater. The toxin has been linked to thyroid irregularities, particularly in women, infants and children younger than 12.

Seven years ago Massachusetts became the first state to set protective standards for perchlorate in drinking water. And, in 2008, at least three perchlorate firework displays around Cape Cod were identified as possible sources of drinking-water contamination. The state Department of Environmental Protection (DEP) issued environmental recommendations “to minimize potential problems” with perchlorate, such as buying no- or low-perchlorate fireworks.

Low-grade backyard fireworks such as sparklers, smoke bombs and ground-based sparking devises also pose risks. These over-the-counter fireworks emit many of the same pollutants, toxins and dioxins as the big shows, and can be just as harmful to people, pets and the environment.

A 2006 report published in Atmospheric Environment concluded that firework displays create potent air pollution. “Although firework-related recreational pollution episodes are transient in nature, they are highly concentrated, contribute significantly to total annual metal emissions, and are on average fine enough to be easily inhaled and a health risk to susceptible individuals,” wrote Roberta Vecchi, author of the study.

Health officials say breathing fine particles in fireworks smoke increases the probability of health problems such as risk of heart attack and stroke, lung inflammation, reduced lung function, asthma-like symptoms and asthma attacks.

The industry trade group American Pyrotechnics Association (APA) says really bad stuff such as lead and mercury are no longer in fireworks. Harmful byproducts, such as copper and barium, burn off when ignited, according to APA director Julie Heckman. “The fireworks we have today are definitely more eco-friendly than we had two decades ago,” she said.

Heckman noted a 2010 study in Lake George, N.Y., that found no perchlorate contamination in the popular vacation destination, a site for hundreds of sanctioned and unsanctioned fireworks shows each summer. She also said unexploded fireworks, or duds, although rare, pose a health concern. Pyrotechnic professionals are taught to check regularly for unexploded fireworks, a practice, she admitted, that is practically impossible over open water.

Officials say duds and used casings, which are mostly made of cardboard and plastic, shouldn’t be recycled. Used casings should go in the trash. Duds need to be brought to your local fire station for proper disposal.