This article has been largely written by AI during a conversation lasting about an hour. It seems to me to make sense and the conclusions appear sound. But it for you, the reader, to judge.
1 Purpose
This article describes the basis for the estimates used for the proportion of four population groups — baby/pre-school child, school-age child, adult and elderly person — who would, at different times of day, either already be within a building or be able to reach a substantial building within a specified period.
The estimates were developed for emergency-planning purposes, particularly circumstances in which rapid sheltering may be advised following an atmospheric release. They are derived planning estimates rather than directly observed population statistics. No UK dataset has been identified that directly records the proportion of the population that is, at a particular time, within 5 or 15 minutes of a suitable building.
The estimates therefore combine evidence on:
- time spent at home and undertaking different activities;
- travel and walking;
- age-dependent travel behaviour;
- children’s outdoor activity;
- outdoor occupations; and
- the accessibility of open and green spaces.
The resulting estimates are reproduced in Table 1.
Table 1. Estimated population location relative to accessible substantial buildings
| Age group | Period | Already inside | Outside but shelter ≤5 min | Shelter 5–15 min | >15 min from shelter | Shelter ≤15 min |
| Baby/pre-school | Night | 99% | 0.5% | 0.3% | 0.2% | 99.8% |
| Working day | 94% | 4% | 1.5% | 0.5% | 99.5% | |
| Evening | 96% | 2.5% | 1.0% | 0.5% | 99.5% | |
| Weekend day | 92% | 5% | 2% | 1% | 99% | |
| Child | Night | 99% | 0.5% | 0.3% | 0.2% | 99.8% |
| School day | 94% | 4% | 1.5% | 0.5% | 99.5% | |
| Evening | 91% | 5% | 2.5% | 1.5% | 98.5% | |
| Weekend day | 85% | 8% | 4% | 3% | 97% | |
| Adult | Night | 98% | 1% | 0.5% | 0.5% | 99.5% |
| Working day | 86% | 7% | 4% | 3% | 97% | |
| Evening | 90% | 5% | 3% | 2% | 98% | |
| Weekend day | 84% | 8% | 5% | 3% | 97% | |
| Elderly (65+) | Night | 99% | 0.5% | 0.3% | 0.2% | 99.8% |
| Weekday day | 93% | 4% | 2% | 1% | 99% | |
| Evening | 96% | 2% | 1.5% | 0.5% | 99.5% | |
| Weekend day | 91% | 5% | 2.5% | 1.5% | 98.5% |
Percentages have been rounded and should not be interpreted as having the statistical precision implied by the individual values.
2 Definition of the location categories
For this assessment a substantial building means a normally occupied or occupiable enclosed building that could provide materially greater protection from an airborne release than remaining outdoors. Houses, schools, offices, shops, public buildings and substantial industrial or commercial premises would normally qualify. Open shelters, lightweight open-sided structures and vehicles have not been assumed to be equivalent to substantial buildings.
Four states were considered:
Already inside: the person is already within a substantial building.
Shelter within 5 minutes: the person is outside but is in a location from which a substantial building could reasonably be entered within approximately five minutes.
Shelter within 5–15 minutes: access is possible but is not immediate.
More than 15 minutes from shelter: the person is in a location or undertaking an activity for which access to a substantial building within approximately 15 minutes cannot reasonably be assumed.
The assessment is consequently not an estimate simply of the fraction of people who are outdoors. This distinction is important. A person walking along an urban street, standing in a school playground or working outside a commercial building is outdoors but may be only seconds or minutes from a substantial building. Such a person is materially different, for emergency-planning purposes, from a hill walker, agricultural worker in a remote field or person on an isolated beach.
3 Adult time-use evidence
The principal evidence concerning the distribution of adult activity through the day was the ONS Time use in the UK: 23 September to 1 October 2023 survey. Participants recorded their activities over complete 24-hour periods. Each participant was allocated one weekday and one weekend diary day. Main activities were recorded in 10-minute periods. The survey therefore provides a useful empirical basis for distinguishing weekday and weekend behaviour and the major components of the day, including sleep/rest, work, household activity, leisure and travel.
The survey does not, however, provide a direct measure of distance or travel time to a suitable building. Time-use evidence was therefore used primarily to constrain the fraction likely to be at home, at work or undertaking other activities, rather than to determine the 5- and 15-minute categories directly.
The very high night-time indoor fractions in Table X.1 follow principally from the dominance of sleep/rest and residential activity during the night. A small residual outdoor fraction was retained to represent night workers, travellers, social/leisure activity and other exceptional circumstances.
Adults were assigned the lowest daytime indoor fraction because this group contains most commuters, outdoor workers and people undertaking travel, shopping, recreation and other activities away from home.
4 Travel
Travel represents one of the principal reasons why a person may not be inside a building at a randomly selected time. The Department for Transport National Travel Survey 2024 (NTS) was therefore used as a second principal evidence source.
The NTS shows substantial travel activity across the population and permits differentiation by age, journey purpose and mode. It was used to constrain the amount of time that could plausibly be assigned to the outdoor and travelling categories.
Travel was not treated as synonymous with inability to shelter. This is an important methodological assumption.
Much travel takes place within built-up areas or between buildings. Similarly, a walking trip does not imply that the pedestrian is remote from shelter. A person walking through a town centre may be outside for an appreciable period while remaining continuously within a minute or two of shops, offices and other buildings.
Consequently, only a fraction of travel activity was allocated to the 5–15-minute category and a still smaller fraction to the >15-minute category. This is a judgement-based allocation rather than an NTS statistic.
5 Children
The principal additional source used for children was the ONS analysis Children’s engagement with the outdoors and sports activities, UK: 2014 to 2015, derived from UK time-use data.
Children aged 8–15 spent an average of 16 minutes per day in parks, countryside, seaside, beach or coastal locations. The corresponding daily participation rate was 12.2%. ONS also found higher average time in these locations at weekends than during weekdays, although the reported difference was not statistically significant.
This evidence was particularly useful because it prevents all children’s leisure or sporting activity from being classified as genuinely remote outdoor activity. Sports and active leisure include activities undertaken indoors and activities undertaken immediately adjacent to buildings.
The child estimates were therefore constructed on the following basis:
- night-time occupancy was assumed to be overwhelmingly residential;
- school-day occupancy was assumed to be dominated by home and school, both providing immediate building access;
- the outdoor fraction was increased during evenings; and
- the largest outdoor and remote fraction was assigned to weekend daytime, reflecting greater opportunity for outdoor recreation, sport and countryside activity.
This produced an estimated 3% >15 minutes from a substantial building during weekend daytime, compared with approximately 0.5% during the school day.
The distinction is deliberately conservative because even participation in a park, countryside or coastal activity does not demonstrate that the participant is more than 15 minutes from a building.
6 Babies and pre-school children
There is less direct activity-location evidence for babies and very young children. Their estimates therefore have greater inferential content.
The principal assumption was that babies and pre-school children spend a greater proportion of their time either at home or in childcare, nursery, retail, hospitality or other locations containing buildings. They are also normally accompanied when travelling or undertaking outdoor activity.
A smaller genuinely remote fraction than for school-age children was consequently assigned.
Weekend daytime was again assigned the largest outdoor component, giving an estimated 1% >15 minutes from a building. The estimate is intended to encompass family countryside recreation, beaches, parks and other activities away from buildings.
No claim is made that the 1% value has been directly measured.
7 Elderly population
Older people were assigned high building-accessibility fractions because employment and commuting become progressively less important with age and a greater proportion of activity is residential.
The NTS age breakdown was used qualitatively in making this judgement because it demonstrates substantial changes in journey purpose and mode with age. It also prevents the erroneous assumption that elderly people are almost continuously at home: shopping, personal business, social activity and recreational walking remain important activities amongst older groups.
Accordingly, the estimates allow an appreciable daytime outdoor/travelling fraction while retaining a comparatively small >15-minute fraction. The estimated latter fraction ranges from approximately 0.2% at night to 1–1.5% during daytime/weekend periods.
8 Outdoor occupations
Outdoor employment was specifically considered because it could otherwise result in an underestimate of the adult weekday exposed population.
HSE identifies occupations involving significant outdoor exposure including farm workers, construction workers, market gardeners, outdoor-activity workers and some public-service workers.
ONS also provides Annual Population Survey employment estimates at four-digit occupational classification and four-digit industry level, including breakdowns by age and country. This confirms that sufficiently detailed employment data exist to identify occupational groups with a significant outdoor component.
Outdoor employment was nevertheless not equated with remoteness from buildings. Conceptually, the contribution from an occupation was considered as:
occupational prevalence × fraction of working time outdoors × fraction of outdoor working time remote from a suitable building.
For example, agricultural and forestry work can place workers appreciable distances from buildings and was therefore regarded as contributing disproportionately to the >15-minute category.
Construction workers may also spend a substantial fraction of their working day outdoors, but construction sites frequently contain completed or partly completed buildings, site offices, welfare accommodation and adjacent development. Their probability of being outdoors is therefore substantially greater than their probability of being more than 15 minutes from any usable building.
This distinction was one reason why a relatively large outdoor component could coexist with the comparatively small 3% adult weekday >15-minute estimate.
9 Open-space accessibility
A further plausibility check was provided by Defra’s 2026 Access to green and blue space in England statistics.
Defra defines its “15-minute commitment” in terms of access by walking route to specified green/blue spaces. It reports that 80% of households in England have access to at least one qualifying green or blue space within a 15-minute walk, increasing to 91% for rural households and decreasing to 78% for urban households.
These statistics cannot simply be inverted to calculate distance from a building. They nevertheless provide a useful spatial plausibility check.
In particular, they demonstrate that a large amount of access to parks, paths and other outdoor space occurs directly from the built environment. Consequently, categorising everyone engaged in outdoor recreation as being remote from buildings would substantially overstate the population unable to shelter rapidly.
The Defra evidence was therefore used qualitatively to constrain, rather than directly calculate, the >15-minute fractions.
10 Construction of the estimates
The entries in Table 1 were produced sequentially.
First, a central estimate was made of the fraction already within buildings, using the time-of-day activity pattern appropriate to each age group.
Second, the residual outdoor/travelling population was divided between locations expected to have immediate building accessibility and locations involving progressively greater separation from buildings.
Third, NTS travel evidence was used to check that the outdoor/travelling fraction was compatible with observed levels of travel.
Fourth, the child estimates were checked against the ONS outdoor-activity evidence.
Fifth, the adult weekday estimate was increased relative to the other groups to accommodate outdoor occupations and work-related travel.
Sixth, the weekend child and adult fractions were increased to represent outdoor recreation.
Finally, the estimates were checked against the Defra spatial-access evidence to ensure that outdoor recreation had not implicitly been treated as synonymous with remoteness from buildings.
The method is therefore best described as a constrained expert-judgement model rather than a statistical derivation.
11 Uncertainty and conservatism
There are two different types of uncertainty in Table 1.
The first concerns the fraction already inside a building. This is reasonably well constrained by time-use, employment and travel evidence, although there remains uncertainty concerning the precise definition of indoor activity.
The second, and substantially larger, uncertainty concerns the division of the outdoor population between the ≤5-minute, 5–15-minute and >15-minute categories. No identified national survey measures these quantities directly.
Consequently, values such as:
Adult, working day: 86% inside, 7% within 5 minutes, 4% within 5–15 minutes and 3% beyond 15 minutes
should not be interpreted as measurements with percentage-point precision. A more appropriate interpretation is that the evidence indicates an indoor fraction of approximately 85–90%, while expert judgement suggests that most of the residual population remains close to buildings and only a few percent is likely to be genuinely remote.
For this reason, where the purpose is emergency-planning modelling rather than demographic analysis, a simpler assumption is preferable.
The detailed analysis indicates central estimates of approximately 97–99+% of the population able to obtain access to a substantial building within 15 minutes, depending upon age and time of day.
A generic assumption of:
95% able to obtain substantial shelter within 15 minutes
was therefore proposed as a deliberately conservative planning value.
This 95% value is not an ONS, DfT, HSE or Defra statistic. It is a modelling assumption derived from the evidence described above and incorporates an allowance for uncertainty.
12 Limitations
The estimates are intended to characterise a generic UK population. They should not automatically be applied to a particular site without considering local circumstances.
The proportion unable to reach shelter rapidly could be higher around locations characterised by extensive:
- agricultural land;
- moorland or mountains;
- beaches and coastline;
- large parks or recreational areas;
- forestry;
- reservoirs and other water-based recreation;
- outdoor industrial activity; or
- transport infrastructure remote from occupied buildings.
Conversely, the generic estimates are likely to be conservative in densely developed urban areas where people outdoors are generally close to multiple buildings.
The analysis also considers physical access to a building, rather than whether the building is open to the public, whether access would be permitted during an emergency, or the degree of radiological protection actually afforded by the building. These issues would require separate consideration.
13 References
- Office for National Statistics (2023). Time use in the UK: 23 September to 1 October 2023. Experimental statistics. The survey used full 24-hour diaries, with participants allocated one weekday and one weekend diary day.
ONS — Time use in the UK - Office for National Statistics. Children’s engagement with the outdoors and sports activities, UK: 2014 to 2015. Includes time and participation rates for children aged 8–15 in parks, countryside and coastal locations.
ONS — Children’s engagement with the outdoors and sports activities - Department for Transport (2025). National Travel Survey 2024. Used for travel frequency, duration, purpose, mode and age-dependent travel behaviour.
- Health and Safety Executive. Outdoor workers and sun exposure. Identifies occupational groups for whom work involves extended periods outdoors, including agricultural and construction workers.
HSE — Outdoor workers and sun exposure - Office for National Statistics (2026). Employment by detailed occupation and industry, by sex, age group and country, 2024 and 2025. Annual Population Survey estimates at four-digit occupational and industry classification.
ONS — Employment by detailed occupation and industry - Department for Environment, Food & Rural Affairs (2026). Access to green and blue space in England. Official statistics in development; provides walking-route-based measures of access to qualifying open spaces, including the 15-minute commitment.
Defra — Access to green and blue space in England
Overall evidential status
The source evidence supports the proposition that most people are either indoors or undertaking activities in locations from which buildings are readily accessible. It does not directly measure building accessibility.
Accordingly, Table 1 should be described as derived planning estimates informed by UK time-use, travel, employment and spatial-access statistics, rather than as measured population fractions. The greatest uncertainty attaches to the allocation of people who are outdoors between the ≤5-minute, 5–15-minute and >15-minute categories.