Pugmire, Sherrill, Venker, Martinez, Guerra (2012)
Respiratory Health Effects Of Childhood Exposure To Environmental Tobacco Smoke In Children Followed To Adulthood(abstract only) is a study that purports to blame SHS for a range of long-term childhood respiratory symptoms: asthma, wheezing (yes, really,) coughing and chronic coughing.
Despite the fact that such illnesses and symptoms increased, while prevalence of smokers decreased in the same (15 year) period, the researchers still managed to find a positive correlation between smoking and the illnesses and symptoms:
Numbers quoted in the abstract follow the form of:
Which suggest that if you didn't have any SHS exposure as a child, ever, you were unlikely likely to have a persistent wheeze as an adult, where as if you had a single whiff of parental SHS you were 1.9 times as likely to have a persistent wheeze in adulthood. (2 times a small number still tends to be a small number, regardless of whether you consider 2-times a small thing 'significant.')
If both your parents were 60-a-day smokers however, and never opened the windows forcing you to breathe in all that SHS, you were.... still 1.9 times as likely to have a persistent wheeze in adulthood.
The press release also contained some interesting data, among which was the atrition rate: 3,805 individuals were enrolled in 1972. By 1996, this had dwindled to 371.
Given that there was no apparent distinction in the abstract between (say) just of the parents admitting to just one cigarette during that period, and both parents smoking 40 a day during the whole period, regardless of whether it was in front of the child(ren) this really isn't surprising. Note that siblings are treated as individual members of the cohort; a family with a single smoking member and triplets would require 3 single-child families where neither parent smoked to 'cancel' out.