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Systems and policies to reduce secondhand smoke in multiunit housing in Singapore: a qualitative study
  1. Yvette van der Eijk,
  2. Grace Ping Ping Tan,
  3. Odelia Teo
  1. Saw Swee Hock School of Public Health, National University of Singapore, Singapore
  1. Correspondence to Dr Yvette van der Eijk, National University of Singapore, Singapore, Singapore; yvette.eijk{at}


Background Multiunit housing residents are often exposed to neighbours’ secondhand smoke (SHS). Little is known on the current systems available to protect residents in places not covered by a residential smoking ban, or what constitutes an appropriate policy approach. This study explores relevant systems and policies in Singapore, a densely populated city-state where the vast majority live in multiunit housing and discussions on regulating smoking in homes are ongoing.

Methods In-depth interviews with 18 key informants involved in thought leadership, advocacy, policy or handling SHS complaints, and 14 smokers and 16 non-smokers exposed to SHS at home.

Results The current system to address neighbours’ SHS comprises three steps: moral suasion, mediation and legal dispute. Moral suasion and mediation are often ineffective as they depend on smokers to willingly restrict their smoking habits. Legal dispute can yield a court order to stop smoking inside the home, but the process places a high evidence burden on complainants. While setting up designated smoking points or running social responsibility campaigns may help to create no-smoking norms, more intractable cases will likely require regulation, a polarising approach which raises concerns about privacy.

Conclusions Without regulations to limit SHS in multiunit housing, current systems are limited in their enforceability as they treat SHS as a neighbourly nuisance rather than a public health threat.

  • environment
  • litigation
  • prevention
  • public policy
  • secondhand smoke

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  • No country has a blanket national-level policy to restrict smoking in multiunit housing, leaving residents unprotected from secondhand smoke (SHS) in their homes. SHS drift from neighbouring units is a common issue and may be associated with respiratory disease symptoms in children.


  • This study explores options available to residents in Singapore where residential SHS is a commonly reported but unregulated issue. These include current options (moral suasion, mediation and legal dispute), which treat residential SHS as a nuisance rather than a serious public health hazard, and potential policy options including social responsibility campaigns, designated smoking areas and regulation.


  • This study provides detailed insights into the various policy options that may be used to address SHS in multiunit housing.


Exposure to tobacco secondhand smoke (SHS) causes lung cancer, cardiovascular disease, asthma and many other diseases, and is harmful even at low levels.1 Although smoke-free legislations protect people from SHS in public places,2 multiunit housing residents are often exposed to SHS from neighbours,3–5 especially in large cities such as Los Angeles,6 New York City7 and Seoul,8 where people live in close proximity to each other. Smoking inside homes is banned in US public multiunit housing,9 all multiunit housing in 67 Californian localities10 and three Canadian provinces.11–13 Australia has set a national recommendation to ban smoking in multiunit housing,14 and in two Australian territories, model by-laws obligate residents to avoid smoking in areas where their SHS can permeate into neighbouring units.15

In Singapore, a densely populated city-state in South-East Asia, 95% of the population live in multiunit housing.16 Despite a low smoking prevalence (11%) and comprehensive smoke-free legislations covering public places,17 18 exposure to SHS from neighbours remains common due to the high population density.19 The Singapore Parliament has debated proposals to regulate smoking in homes since 2017 due to high volumes of complaints about SHS from neighbours, exacerbated by the COVID-19 pandemic stay-at-home measures.19 20 In 2017, two Members of Parliament (MPs) proposed to set a limit for residential SHS exposure above which a court order could be issued to the neighbour.19 In 2018–2021, an MP proposed to ban smoking at the windows and balconies of multiunit housing.20

While these proposals were met with lively debates21 22 and majority support from the public,23 they were also controversial, with concerns about privacy and enforcement. Hence the Singapore Government has opted for a softer approach, by encouraging social responsibility and committing to improve existing channels for residents’ dispute resolution.24 In addition, two districts installed designated smoking points (DSPs) in public housing estates,25 26 and some private condominiums attempted to pass by-laws banning smoking at patios, balconies and windows of individual units.27 However, in the absence of national regulations, SHS remains a potential health threat for multiunit housing residents.

Currently, there are no international guidelines for policies to limit SHS in multiunit housing. Given the contention of the issue, it is unclear whether the appropriate policy constitutes a full smoking ban or middle ground approach, such as that in Singapore. Little is known on current systems available to protect residents from neighbours’ SHS in places not covered by a smoking ban. Hence our research questions were as follows: (1) With no regulation, what options are available to affected residents? (2) How should the issue be dealt with? We explored these from the perspective of Singapore smokers, affected non-smokers and experts with a background in advocacy, policy, thought leadership or handling residential SHS complaints.


In March-August 2021, we conducted in-depth interviews with 18 key informant experts (table 1), and 30 Singapore residents including 14 current smokers and 16 non-smokers exposed to SHS in their home (table 2). All participants were English-speaking, Singaporean (citizen or permanent resident) adults (age 21 years and above). We recruited participants via email invitation, flyers and snowball methods. Prior to joining the study, residents provided information on their sociodemographics, smoking habits and history of residential SHS exposure to enable sample balancing in terms of age, gender, ethnicity, housing type, smoking status and experiences with inhome SHS.

Table 1

Details of key informant interviewees

Table 2

Details of resident interviewees

Interviews with key informants were one-on-one while interviews with residents were one-on-one or dyadic. Interviews lasted 40–70 min each, were conducted in English, and were done either in person (n=1) or online using Zoom conferencing (n=47). Interview questions followed an open-ended format, with questions about their views and experiences with systems to address residential SHS and perspective on different interventions (box 1). We reimbursed each resident or dyad with $S50 cash.

Box 1

Interview guide for residents and key informants


Background information

  1. Please tell me about the people you live with and your/their smoking habits.

  2. Please tell me about your experiences with secondhand smoke at home.

Addressing secondhand smoke from neighbours (for non-smokers)

  1. Did you ever attempt to solve the issue? What was your experience?

  2. Do you know of any platforms available to address secondhand smoke from neighbours? Please describe.

Views on different interventions

  1. Do you think more should be done to address the issue of residential secondhand smoke? Why (not)?

  2. What are your views on having alternative smoking areas in residential areas?

  3. What are your views on regulating smoking in the home to protect neighbours?

  4. What are your views on public education campaigns that focus on promoting social responsibility among smokers?

Key informants

  1. What are the existing mechanisms for addressing the issue of residential secondhand smoke? What are your views on them?

  2. Do you think more should be done to address the issue of residential secondhand smoke? Why (not)?

  3. What are your views on having alternative smoking areas in residential areas?

  4. What are your views on regulating smoking in the home to protect neighbours?

  5. What are your views on public education campaigns that focus on promoting social responsibility among smokers?

Interviews were transcribed verbatim and imported into NVivo. We developed an initial codebook with deductive codes originating from a priori topics in the interview guides, and subsequently modified the codebook to include inductive codes that arose from the data. Transcripts were double-coded and compared among the researchers working independently to ensure coding consistency and establish inter-rater reliability. Similar codes were combined and new codes were added to the codebook during the coding process. Discrepancies were reviewed and discussed by the researchers until consensus was reached. Codes were then organised into categories, subcategories and overarching themes (see online supplemental file for codebook). Some participants sent additional data after the interview, such as statistics, which we incorporated into our results.


The current system to address neighbours’ SHS is a three-step process: (1) Moral suasion, in which neighbours resolve the issue among themselves; (2) Mediation, a formal but non-legally binding process; and (3) Legal dispute at the Community Disputes Resolution Tribunal (CDRT). Interviewees discussed three policy options: DSPs, social responsibility campaigns and regulation.

Complaints procedure for residents

For public housing residents, complaints about a neighbour’s SHS are made to the town council or MP, who encourage neighbours to resolve the issue among themselves. They may also refer them to the Housing and Development Board (HDB), which owns public housing in Singapore, or National Environment Agency (NEA), which enforces smoke-free legislations. Town councils, HDB and NEA have a joint advisory asking smokers to smoke in a socially responsible manner, and typically respond by posting the advisory to units in the affected area. This advisory is not legally binding and, according to key informants, often ignored by smokers. Some MPs also visit the home of the smoker to work out an amicable solution. Intractable cases are then referred to mediation or, failing that, CDRT.

Key informants and non-smoking residents who had experienced this process felt that, without regulation, it is ineffective as it depends on the goodwill of the neighbour in question:

There is quite a few layers of help we can provide from the RC [Residents’ Committee] to the HDB to NEA to the town council, to grassroots helping. But I found that almost all the time, it doesn't really work again because there is no legislation in place. (Policymaker)

In private condos, complaints about a neighbour’s SHS are made to the management who typically respond by visiting the neighbour or posting advisories. Key informants and non-smoking residents living in condos described condo managements as limited in their ability to resolve SHS complaints as, without a regulation or specific bylaw, they were reliant on moral suasion.

Condo by-laws to regulate smoking in homes

Key informants described how condos can pass by-laws to regulate smoking inside homes with a 75% resident vote. As at October 2021, at least four condos in Singapore had passed such by-laws, and two condos had attempted but fell short of the vote. A limitation of these by-laws is that they cannot be enforced with a substantial fine. By-laws are enforceable in court but at a high financial cost:

Bylaws are passed, but… the issue is enforcement. We may have a law but no one wants to enforce. You tell them (residents), it will cost you $2000 to get into court. They go, ‘Huh? Must pay money… then (they) don’t want (to).’ (Legal expert)

By-laws can, however, be used to create a no-smoking norm in the condo, for instance by asking new residents to declare their smoking status at registration and referring current smokers to the no-smoking by-law. By-laws may also apply an administration fee for each violation which, although not intended to act as a fine, may deter some smokers.


Key informants and non-smoking residents described mediation at the Community Mediation Centre (CMC) as the next step for cases not resolved amicably. The smoking neighbour is not legally required to attend, and the majority do not. From 2016 to 2019, CMC received 148 applications or referrals for cases related to SHS disputes. Of these, only 14% were mediated.

According to legal experts, the goal of mediation is not to objectively determine who is at fault, but to enable both parties to share their views and come up with an amicable solution. This requires a willingness on both sides to resolve the issue:

We always believe that everything can be mediated, but that’s on the assumption that you want to resolve the issue. You must be prepared to settle or give something in return for something else. (Legal expert)

If resolved, parties sign a settlement agreement which lays out specific terms for both parties, such as measures the smoker will take to minimise SHS. This is not a legally binding court order but a contract which, if violated, can be enforced, although this is costly as it requires hiring a lawyer and going to court. Legal experts described mediation as an easier and more amicable process than litigation:

Mediation helps you to understand the other party’s issues, it is a bit cheaper. It’s less time-consuming, and you know the outcome you want if you agree… For litigation, you will not know what’s the outcome until the court gives you the order. No matter what you spend, there are only two outcomes, you win or you lose. (Legal expert)

However, key informants and non-smoking residents strongly felt that mediation is limited by the fact that it is voluntary. Most neighbours responsible for the SHS refuse to go as there is no penalty in doing so. Mandatory mediation was not considered a viable option:

…it has to be voluntary because if you force someone to come down there to try and resolve the matter, unlikely they're going to do it… they do it as a box-ticking exercise: ‘I just show up. I go through the mediation. I come back and say I’ve tried it…’. (Legal expert)

They also noted that, since mediation requires both neighbours to be invested in resolving the issue, it only works for those who do not need mediation in the first place. Participants who had tried mediation for SHS disputes noted that, although their cases were settled, it had led to limited improvements in their situation, mainly due to the practical difficulties in enforcing the settlement agreement.

Legal dispute

Key informants described CDRT, a legal avenue for intractable neighbour disputes involving interferences that affect one’s enjoyment of residential spaces, such as excessive noise and smell. As the process is intended to be fast and financially accessible, residents represent themselves. Unlike mediation, both parties are legally required to attend CDRT. Judges can order parties to attend mandatory mediation or issue a court order to restrict smoking.

To make a case, plaintiffs must gather evidence to show that SHS is frequently entering their home and coming from the respondent’s unit. Plaintiffs may also gather evidence of any SHS-related health conditions and attempts to resolve the issue amicably. An advocate who had won a CDRT case for an SHS dispute had kept meticulous records of SHS incursions into the home, interactions with the neighbour and medical reports of health issues linked to SHS. The advocate had also spoken with other neighbours to rule out any other units as the SHS source. A legal expert described how, in addition to evidence, the ruling usually takes into account whether the plaintiff’s request is reasonable factoring in the unique situation of both parties:

The solution that a judge can order is not going to affect a person’s day-to-day life in a sense… (smoking is) not something that you have to do at home, or you regularly would do, and so the inconvenience (of regulating it) is less or the intrusion is less. (Legal expert)

For the advocate who won the CDRT case, the neighbour had been ordered to close the windows when smoking inside the home. Although this improved the situation, it did not eliminate the SHS:

…if I'm completely honest, it was on and off, I still smelled the cigarette smoke. (Advocate)

When asked about their perceptions of CDRT, most key informants described it as a tiring process which might aggravate conflict between neighbours. Key informants less familiar with CDRT were unclear on whether SHS from neighbours fell under the scope of CDRT. In contrast, the legal experts and advocate who had won the CDRT case described it as a fast and affordable process, although they also highlighted the uncertainty of the outcome which depended on various factors such as the unique situations of both parties and strength of the evidence presented.

Regulating smoking in homes

Key informants and residents, including some smokers, felt that smoking in homes should be regulated given the severe health threat posed by SHS and ineffectiveness of current systems. They also viewed it as necessary to send the right message and provide a means to deal with more intractable cases:

I think the law has to step in. it has to come from top-down, it can’t be personal, right?… It is a public health issue and it has to come from the government. (Resident (non-smoker))

Smokers also noted how a regulation might encourage them to quit. Hence they felt that a regulation should be complemented with other interventions such as DSPs or quit support. However, many participants, especially smokers, felt that regulation would be too excessive. Smokers viewed it as an undue invasion of privacy, while key informants and non-smokers reasoned that it would be more appropriate to start with less intrusive interventions:

I'm more in favour of the softer approach because I think that’s more sustainable and it doesn't make people resent the rules. (Legal expert)

Opinions were mixed on whether banning smoking at balconies and windows would work. Some participants believed that people would just need time to adjust, while others felt it would be unfeasible altogether:

If they stop my smoking in my house, then where can I go to? So everybody, we are standing outside the corridor, we’ll be downstairs smoking. (Resident (smoker))

Some smokers stated that they would smoke with the windows closed, exposing their family members to SHS, or would smoke out the window in places where it is easier to hide. In one case, the smoker had a mobility issue making a ban practically difficult. Key informants and residents also felt that enforcement would be problematic:

Let’s say 10% of the people smoke, out of a block of maybe two hundred units, you have 20 units where people are smoking inside. How much resources will that entail for the government to check in these units every day and enforce against them? (Public officer)

What exactly is the distance you need to keep from the window, for example? Or if your balcony’s humongous, vs a tiny like yard balcony or something like that. How are you going to establish the parameters across a wide range of different types of housing situations to apply enforcement equally, to make it universal? (Resident (non-smoker))

Key informants and residents raised the possibility of using CCTV as an enforcement mechanism. Some viewed it as an appropriate method that does not invade privacy as CCTV is already used to track high-rise littering, while others viewed it as a violation of privacy:

…it crosses over into a very different territory where privacy starts to play in… to have a monitoring device pointed at my house… (Resident (smoker))

They also discussed the use of crowdsourcing to identify offending units. Some believed that this would be helpful in identifying the SHS source, while others felt that it would result in neighbours naming and shaming each other, causing tension.

Tobacco regulations more broadly

Key informants and residents discussed approaches to minimise SHS in homes within the broader context of tobacco regulations such as taxes, plain packaging and smoking bans in public places. They generally felt that these regulations played an important role in bringing down overall smoking prevalence and shaping social norms, and thereby the issue of SHS in homes:

It does hone the awareness that secondhand smoke actually does…it actually impacts other non-smokers… (Resident (smoker))

Interventions to encourage social responsibility

Key informants and residents discussed options to encourage socially responsible smoking, including campaigns to denormalise smoking in homes, public education on the health effects of SHS or having community leaders engage with smokers directly:

How it could work could be getting like grassroots leaders and other community leaders involved to talk to neighbours and trying to get them to see each other’s point of view and to work out solutions that are customised to that situation. (Public officer)

Compared with legislation, they felt that this approach was a less coercive way to address the issue:

I don't think it’s always about law going after people, right. But it’s also about trying to get people to internalise what is it to be good, what it takes to be, I mean, a good neighbour, a good citizen, you know, and just be a good human being. (Academic)

I wanted to do enough of the moral suasion because if we don't try, there will always be those people who say, Well, look, you haven't tried persuading, you've not persuaded hard enough. (Policy maker)

Yet key informants and smokers were sceptical that such campaigns would work as, in their view, socially responsible smokers already take steps to reduce SHS while less socially responsible individuals are unlikely to respond, requiring a more top-down approach:

It sounds good in theory, but may not actually translate into much action because this mutual understanding thing, it’s really very vague. (Doctor)

We have to respond to the stick rather than the carrot. Public education has not really worked. (Resident (smokers))

Designated smoking points

Key informants described DSP initiatives in two public housing districts, Nee Soon South and Clementi. In 2014, Nee Soon South introduced a community smoke-free zone with six DSPs, open-air shelters with benches to sit, as a 1 year pilot. After a successful pilot, the initiative was scaled up to include one DSP per every two to three blocks in the district. In 2021, Clementi piloted two DSPs: an enclosed cabin fitted with air-conditioning and air-filtration systems, and an open-air cabin with an eco-friendly cooling system. The rationale for these added comfort features was to incentivise smokers to use the DSP.

Key informants felt that DSPs are beneficial as they provide an alternative for smokers, make it easier for non-smokers to avoid SHS and can create positive no-smoking norms:

It’s not that there aren't smokers but the smokers do use the DSP and they encourage each other to use the DSP. I think that’s a good norm. (Policy maker)

A key informant described how having DSPs could also strengthen a resident’s legal case:

If there is a DSP in the neighbourhood, then the presumption would be that actually, you could have used it but you chose not to use it. Therefore, the judge in the CDRT could be more disposed towards thinking that there needs to be some restrictions placed on you because there are alternatives which you didn't use and which if you had used this case wouldn't come to CDRT. (Policy maker)

Smokers felt that DSPs would benefit them, with some framing it as a matter of reciprocity if more aggressive measures were to be implemented:

I would use it. No more secondhand smoke for my family member, and at the same time I can cut down my smoking. (Resident (smoker))

…as a smoker, you must give breaks for people to, area to smoke also. You want to stop certain thing, you need to have something for them also. (Resident (smoker))

Some smokers expressed concerns about the DSPs becoming overcrowded. Smokers, key informants and non-smokers also expressed doubts that smokers would use DSPs due to the inconvenience:

If it is inconvenient then I probably wouldn't do it at all. Right now if you ask me, even if it’s just downstairs, just go downstairs and it is just there. It is, for me it is already such a hassle. (Resident (smoker))

Yet they also believed that smokers would use DSPs if smoking inside the home was regulated. Smokers who were opposed to voluntarily using the DSPs also held this view:

Well, if I don't have any other choice, definitely I will have to use them. (Resident (smoker))

Key informants and non-smoking residents noted that DSPs might present a nuisance to people living nearby or have high installation and maintenance costs. At the same time, they believed that it was important to make the DSPs appealing to smokers:

If you want it to attract the smokers, you have to make it attractive… If you think of this as a luxury for smokers, then we're never going to get anywhere because you will have all kinds of philosophical objections. (Policy maker

In terms of design, smokers were not keen on enclosed DSPs as they found the smell of others’ SHS unpleasant. Some also described enclosed DSPs as having a claustrophobic feel. They did not like fully open-air DSPs as they have no protection from the weather. For most smokers, the optimal design was an open-air shelter with a place to sit.


SHS exposure in multiunit housing is not merely a nuisance, but a public health hazard that pervades into residents’ personal life. Interventions require smokers to modify their personal habits, while not intervening potentially leaves others exposed to a serious health threat inside their own home. It is therefore a contentious issue that, in both the presence or absence of intervention, tends to result in polarised views as observed in our participants. We also observed smokers as falling on a spectrum, with some amenable to ‘soft’ interventions such as moral suasion and others likely to respond only to well-enforced regulations.

The intervention ladder of public health ethics conceptualises interventions in terms of their intrusiveness, with more intrusive ones requiring a stronger justification (table 3).28 Policies to minimise SHS in multiunit housing may be framed in this way, with less intrusive interventions (moral suasion, DSPs) as the first-line approach, followed with middle ground interventions (social norms reshaping, social responsibility campaigns, mediation) and, if the issue persists, regulation. This could be a full smoking ban in multiunit housing, as implemented in parts of the USA and Canada, or a partial ban as proposed in Singapore. Although court orders from residential disputes and condo by-laws aim to restrict or eliminate choice, challenges in enforcement means that, in reality, they act more as a disincentive. Given the addictiveness of smoking, interventions that enable people to not smoke in their homes, such as quit support and DSPs, complement regulations that restrict or eliminate choice.

Table 3

Interventions to address secondhand smoke in multiunit housing conceptualised along the intervention ladder of public health ethics, ordered from the most intrusive (top) to the least intrusive (bottom)28

Current systems, which treat residential SHS as a neighbourly nuisance, fall on the lowest rungs of the intervention ladder (table 3). The first-line approach, moral suasion, was considered ineffective as it depends on smokers voluntarily modifying their smoking habits to benefit neighbours. Hence moral suasion is limited by factors such as the quality of the neighbourly relationship. For the same reason, mediation, which is voluntary, was considered ineffective. DSPs go a step further by providing smokers with an alternative, but also depend on smokers voluntarily using them. Our smoking participants had varying attitudes in this respect. We also found that certain DSP designs, such as enclosed or unsheltered structures, may deter smokers. Hence it is important to involve smokers in DSP development and to evaluate DSPs from their perspective.29 Although they should be conveniently located, they should be situated some distance from the nearest apartment block as SHS tends to leak out even from enclosed DSPs.30 31

Participants had mixed sentiments on social responsibility campaigns. Some viewed them as a useful, non-coercive way to create smoke-free norms while others were sceptical that they would work. Mass media campaigns that shape public knowledge and behaviour around SHS can help to promote smoke-free homes,32 increase support for smoking bans33–35 and create no-smoking norms.34 However, they have also been used to socialise smoking, as with the tobacco industry’s ‘mutual tolerance’ campaigns which sought to undermine smoke-free legislations.36–38 Underlying messages of campaigns should advocate for smoke-free homes rather than a tolerance of SHS.

Legal dispute is a last-resort option for residents not covered by smoke-free housing rules. Similar legal mechanisms have been described in the context of SHS disputes in Taiwan39 40 and Australia.41 Participants viewed this as a viable but difficult option due to the high evidence burden placed on plaintiffs. SHS exposure is difficult to record unless one has access to biomarker (eg, cotinine) tests or air pollutant monitors. The SHS source can be difficult to pinpoint, especially if SHS is coming from multiple units. Even if the plaintiff successfully obtains a court order, enforcement remains an issue. Thus legal dispute, although it may act as a deterrent, will likely not benefit the majority of cases and may require more streamlined evidence-gathering and enforcement processes.

The most intrusive policy, regulation, was the most polarising. While participants viewed it as a potential invasion of privacy, they also considered it as necessary given the serious health threats posed by SHS and limitations of current systems. Some smoking participants believed that a smoking ban in homes would encourage them to quit, suggesting that it may yield population health benefits. This is consistent with evaluation studies which found reduced smoking rates and increased quit attempts following the implementation of smoke-free multiunit housing policies.42–44 We also found that enforcement may be practically difficult in the home environment, and compliance may be challenging for some smokers, such as those with mobility issues. Proper enforcement is crucial to the success of smoking bans, including in the home environment. It is important to consider the variety of enforcement and support measures, such as public education campaigns, that influence the success of smoking regulations.45–47


As our data are qualitative, it cannot be used to generalise at the population level. As our study was conducted during the COVID-19 pandemic, our sample may under-represent certain groups, such as participants with limited online access.


With no regulation, recourse mechanisms for residents affected by a neighbour’s SHS are limited as they treat SHS as a neighbourly nuisance rather than a serious public health threat. While middle ground options, such as DSPs and social responsibility campaigns, may help to create no-smoking norms, regulation is the only intervention likely to effectively protect residents.

Data availability statement

No data are available.

Ethics statements

Patient consent for publication

Ethics approval

This study involves human participants and was approved by National University of Singapore Institutional Review Board (reference NUS-IRB-2021-79) Participants gave informed consent to participate in the study before taking part.



  • Contributors YvdE: study conceptualisation, study design, data analysis, writing. GPPT: data collection, data analysis, writing. OT: data collection, data analysis. All authors reviewed and approved the final draft before submission. YvdE is the guarantor.

  • Funding This work was supported by a Tier 1 Academic Research Fund from the Singapore Ministry of Education (grant number R-608-000-302-114) and a start-up fund from the National University of Singapore (grant number R-608-000-303-133).

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.