Exposure to secondhand smoke in the US has dropped by half between 1999 and 2012. However, 1 in 4 nonsmokers are still exposed to deadly secondhand smoke, according to a new study.

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A total of 26 states and the District of Colombia have implemented comprehensive smoke-free legislation since 2002.

The report from the Centers for Disease Control and Protection (CDC) states that around 58 million nonsmokers and 2 out of 5 children aged 3-11 years are still exposed despite a significant reduction in secondhand smoke levels.

“Secondhand smoke can kill. Too many Americans, and especially too many American children, are still exposed to it,” says CDC Director Dr. Tom Frieden. “That 40% of children – including 7 in 10 black children – are still exposed shows how much more we have to do to protect everyone from this preventable health hazard.”

SHS exposure was assessed by measuring the levels of cotinine – a chemical compound found in tobacco – in the blood. Data were obtained from the National Health and Nutrition Examination Survey.

The CDC state that there is no safe level of secondhand smoke (SHS) exposure. Every year, SHS is associated with the deaths of over 400 infants and 41,000 adult nonsmokers.

SHS contains over 7,000 chemicals, of which experts have identified 70 that can cause cancer. Among infants and children, SHS can lead to asthma attacks, ear infections, respiratory infections and Sudden Infant Death Syndrome (SIDS). Among adults, it can also lead to heart disease and stroke.

Although SHS exposure levels have declined overall, certain demographic groups are affected more than others. Nonsmokers most likely to be affected by SHS are black people (nearly half), children aged 3-11, people below the poverty line (2 in 5) and people living in rented accommodation (1 in 3).

In the report, rental status is used to identify people living in multiunit housing. Around 80 million Americans are estimated to live in multiunit housing, where secondhand smoke can seep into nonsmoking homes and shared areas from other places where smoking occurs.

“The potential of exposure in subsidized housing is especially concerning because many of the residents – including children, the elderly, and people with disabilities – are particularly sensitive to the effects of secondhand smoke,” states Brian King, acting deputy director for research translation in CDC’s Office on Smoking and Health.

The CDC credit several factors for the observed decline in SHS exposure levels. Almost half of the US population (49.3%) is now covered by local and state laws prohibiting smoking in public areas such as workplaces, restaurants and bars.

A total of 26 states, the District of Colombia and nearly 700 cities have passed strong smoke-free laws. A growing number of households have also instigated smoke-free rules, rising from 43% in 1992-93 to 83% in 2010-11.

In addition, the CDC report that cigarette smoking has become less popular over the last 20 years and smoking around nonsmokers less socially acceptable.

Despite these improvements, any level of exposure to SHS can be hazardous, and further action can be taken to reduce these figures further.

The authors of the report state that fully protecting nonsmokers from SHS exposure requires the elimination of smoking from indoor spaces:

Continued efforts to promote implementation of comprehensive statewide laws prohibiting smoking in workplaces and public places, smoke-free policies in multiunit housing, and voluntary smoke-free home and vehicle rules are critical to protect nonsmokers from this preventable health hazard in the places they live, work and gather.”

The CDC Vital Signs report appears as part of the journal Morbidity and Mortality Week Report.

Last year, Medical News Today reported on a study suggesting that nonsmokers who live with people who smoke are exposed to three times the healthy limit of dangerous air particles, at a level comparable to those living in heavily polluted cities.