Children Learning About Second-hand Smoke (CLASS II): A pilot cluster randomised controlled trial Breathing in other people's smoke is called passive smoking; also sometimes referred to as involuntary or second-hand smoking (SHS). Second-hand smoke contains 4000 chemicals, 70 of which can cause cancer. SHS is particularly harmful to children's health and can lead to chest and ear infections, tuberculosis, meningitis and asthma. It is also associated with lung cancer and heart disease. Globally, 40% of children are exposed to SHS. Many countries have introduced bans on smoking in enclosed public spaces, which has significantly reduced adults' exposure to SHS. However, for the majority of children, cars and homes remain the most likely places for them to breathe in SHS. The only possible way to protect children from SHS is to make cars and homes completely smoke free.   For the last three years, we have been working with teachers, children and their parents in primary schools in Dhaka, Bangladesh to develop and test an intervention called, 'Smoke Free Homes'. It consists of six teaching lessons delivered by schoolteachers, four fun activities and one educational take home resource. Teaching lessons help to increase pupils' knowledge about the harms caused by breathing in other people's smoke. Fun activities include storytelling, role-playing, quizzes and games. These activities help to motivate children to act and feel confident in talking to adults to persuade them not to smoke inside homes. The take-home resource helps children to show what they have learned in school and to negotiate with their families to "sign-up" to a voluntary contract to make their homes smoke-free. The results of this work show that it is possible to encourage children to discuss with their families ways of restricting smoking inside their homes.   Inspired by what we found, we now wish to plan for a large study where we randomly select half of the schools to either have 'Smoke Free Homes' and half not to have it (but to receive it at the end of the study). We wish to examine how effective 'Smoke Free Homes' is in reducing children's exposure to SHS. We are also interested to see if the intervention improves their lung health, general quality of life, school attendance and school performance. We would also like to examine if it helps in changing their attitude towards smoking and makes it less likely for them to take up smoking in future. To provide accurate answers to these important questions, we will need to recruit many schools and possibly thousands of children. We will also need to use objective measurements including testing children's saliva for cotinine - a chemical derivative of nicotine found in those who are exposed to SHS. Other measures will include breathing tests to assess children's lung capacities and volumes, questionnaires and diaries to be kept by children to record their symptoms such as cough, wheeze and scales to measure school performance and attitudes to smoking. Before carrying out this large study, we plan to conduct a pilot study. This pilot study will simulate the large study but will be conducted on a smaller scale. The information obtained from the pilot study will help us to carefully plan for the large study and help us to test our assumptions. These include the number of schools and children to be recruited, feasibility and acceptability of the specified measurements and resource requirement for scaling up the intervention.   We are proposing to conduct this pilot study in Dhaka Division, Bangladesh. We will recruit a total of 12 primary schools and 360, year 5 (10-12 years old) children. We will undertake all the measurements as described above using internationally agreed standards and measures. Once measurements have been taken, half of the schools will be randomly chosen to receive 'Smoke Free Homes' while the other half will not receive the intervention until the end. We will repeat the assessments at two, six and twelve months after the intervention. Findings will help us to plan for the large study. The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.