When the rockets and the bombs burst in the air tonight, spectators will experience more than a spectacular show celebrating America’s birthday.

Chemists say fireworks are the dirtiest of the dirty bombs: When their blends of black powder, metals, oxidizers, fuels and other toxic ingredients are ignited, traces wind up in the environment, often spreading long distances and lasting for days, even months.

Although pyrotechnic experts are developing environmentally friendly fireworks, Fourth of July revelers this year will be watching essentially the same, high-polluting technology that their grandparents experienced decades ago.

Public health officials warn that people with heart problems or respiratory diseases, such as asthma, should avoid the smoky celebrations, staying upwind or indoors.

“I enjoy a fireworks display as much as anyone else but we do have concerns about exposure to high levels of smoke and particles,” said Jean Ospital, health effects officer for the South Coast Air Quality Management District.

Traces of poisonous metals, which give fireworks their bright colors, and perchlorate, a hormone-altering substance used as an oxidizer, contaminate waterways. One Environmental Protection Agency study found that perchlorate levels in an Oklahoma lake rose 1,000-fold after a fireworks display, and they stayed high in some areas for up to 80 days.