How harmful is one hour of exposure to second hand smoke?

Difficult to measure. Exposure to second hand smoke can trigger heart attack in susceptible individuals. How harmful, is difficult to state, as most people do not suffer long term consequences from a single short exposure.
One hour? Minimal. The more you are exposed, the more chance for infections.

Related Questions

Can someone tell me if second hand smoke harmful to my child if I do it only for one day?

Smoke somewhere else. Smoking where a child can breathe in second-hand smoke is not healthy for that child, even once. Certainly, long-term exposure is very unhealthy, with increased colds, sinus infections, asthma and allergy, but even short exposures can cause coughing and allergic reactions. If you are not dependent on tobacco, why are you doing this, even for a day? If you must, do it outside. Read more...
It lingers. Studies have shown the breakdown products of smoke in a babies urine 2 weeks after its only exposure. The particulate matter breathed into the nostrils will attach to the nasal membrane and remain until shed/washed out. Exposure doubles risk of upper respiratory illness. Read more...

Please help docs! Is second hand smoke harmful to my child if only for one day?

To some degree. A study years ago showed break down products of nicotine in the urine of 2 week old babies with no known exposure to smokers. This shows the persistence of smoker derived irritant particles that end up in babies nose. It is surely of no health benefit, but chance exposure to minor levels are part of life. Read more...
Yes but. The ideal answer is yes since any tobacco smoke is unhealthy for you. If you must smoke one cigarette a day. When smoke in the presence of your child at all? The baby can be harmed even before it was born if the mother smokes. Read more...