Frontline paramedics working in the prehospital setting undertake visually demanding tasks in challenging environments, including driving under emergency conditions, assessing patients and implementing lifesaving treatment and interventions at variable light levels. The visual conditions within which these tasks must be undertaken are often far from optimal, and the success of critical tasks may be adversely affected by low luminance levels in dark surroundings and while working at night.
It is widely known that visual function decreases as light levels fall (Hiraoka et al, 2015). However, how this affects the ability of paramedics to undertake their role is hitherto unexplored. There are also no standardised regulations for provision of lighting equipment to support staff across different ambulance settings.
This study explored the impact of working in decreased light levels on paramedic practice and discussed modifications to reduce risk.
The review also aimed to identify the scope for evidence-based occupational standards and guidance for paramedic practice in low lighting conditions by identifying the key aspects of visual function that are affected.
Methods
Professional panel
Twitter was used to recruit prehospital paramedics to a professional panel to provide their opinions and discuss their experiences of working in low light conditions.
Discussions took place via direct messaging or email over 11–21 December 2020. Responses were recorded verbatim. No further probing to clarify or expand answers was undertaken. Transcripts were analysed and themes identified, which then informed the direction of the scoping review.
Scoping review
A literature search was undertaken using Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) scoping methodology (Munn et al, 2018; Tricco et al, 2018). Embase and Medline were chosen, along with Google Scholar. The search strategy is shown in Table 1.
| Theme 1. Driving | ((‘driving’ OR ‘crash’) AND (‘mesopic’ OR ‘low light’ OR ‘low luminance’) OR (‘night myopia’ OR ‘accommodation’ OR ‘aberration’) OR (‘visual deficit’ OR ‘visual impairment’) OR (‘age’ OR ‘glare’)) |
| Theme 2. Glare | ((‘driving’ OR ‘crash’) AND ((‘glare’ OR ‘glare sensitivity’ OR ‘disability glare’ OR ‘discomfort glare’ OR ‘glare enhancement’) OR (‘refractive surgery’ OR ‘LASIK’ OR ‘PRK’ OR ‘cataract’))) |
| Theme 3. Colour | ((‘mesopic’ OR ‘low light’ OR ‘low luminance’) AND ((‘colour’ OR ‘colour vision’ OR ‘colour spectrum’) OR (‘Purkinje shift’ OR ‘colour-blind’))) |
| Theme 4. Visual acuity | ((‘mesopic’ OR ‘scotopic’ OR ‘low light’ OR ‘low luminance’) AND (‘visual acuity’ OR ‘hyperacuity’ OR ‘visual acuity testing’)) |
| Theme 5. Contrast sensitivity | ((‘mesopic’ OR ‘scotopic’ OR ‘low light’ OR ‘low luminance’) AND (‘contrast sensitivity’ OR ‘contrast sensitivity testing’) AND (‘age’ OR ‘optical health’ OR ‘optical disease’ OR ‘colour’)) |
Peer-reviewed publications of any methodological design were considered eligible if they related to paramedic practice, emergency services, vision, visual acuity (VA) and the subject themes identified by the focus group. Publications were excluded if the population was not relevant to paramedics (animal studies, human populations that were outside paramedic working age i.e. aged <18 or >65 years). As this was a scoping review, no age limits were applied to publication date to ensure the breadth of literature could be explored. The full inclusion and exclusion criteria are shown in Table 2. Study findings were collated from included papers using Microsoft Excel.
| Inclusion | Exclusion |
|---|---|
| Human participant |
Animal studies |
Meta-analysis
To further understand the effect of luminance levels on VA, data from studies assessing VA under different light levels were collated and a meta-analysis undertaken.
Results
Professional panel
Five UK registered paramedics engaged with the social media call and took part in the consultation. While engagement was in a personal capacity, all respondents practised in the same ambulance service NHS trust and all undertook frontline work for at least part of their role.
Four respondents identified their gender as male and one identified as female. Wider demographic characteristics were not recorded as this was not part of the intended study design.
Figure 1 shows the areas of practice that are most affected by low-luminance working conditions. These were broadly grouped into two themes: driving; and patient assessment/treatment.
The professional panel discussed changes to the working environment and equipment/technology provisions that could address the difficulties experienced in low-luminance working conditions.
Driving
Driving under emergency conditions and transporting patients to hospital are key features of prehospital care. This became harder at night. In relation to tasks associated with arriving at a scene after nightfall, the panel reported increased difficulties with house number identification, assessment of scene safety and locating patients.
Patient assessment/treatment
The clinical severity of patients presenting to ambulance services is highly variable, but many are critically unwell. The professional panel identified that working in low-luminance conditions made assessing and treating patients more difficult.
Specific tasks affected by lower luminance included detection of skin colour abnormalities, recognising social cues and reading text on equipment and medication. These are all essential tasks that may negatively affect the treatment provided to patients if not done accurately. However, any relationship between this and patient outcomes is unexplored.
The panel also highlighted that provision of lighting equipment is unregulated and varies between both ambulance trusts and vehicles.
Suggestions for improvement
Suggestions from the professional panel to improve driving were limited. The use of a dark mode to reduce the glare from the mobile data terminal was suggested as was using the low-power mode for blue lights at night-time, which may reduce ambient light reflections. Encouraging the public to make house numbers more visible was also suggested.
Increasing scene luminance was considered the most important way to reduce difficulties in assessing and treating patients. The panel suggested this could be best achieved by improving the availability, performance and regulation of both vehicle-based and personal-issue lighting equipment. Portable devices such as hand and head torches and portable scene lighting, as well as fixed equipment on ambulance vehicles, were recommended.
Literature review
One hundred and thirty-seven papers were identified through searching databases (Figure 2). After screening against inclusion and exclusion criteria (Table 2), sixty relevant full-text studies were identified.
The explanations for increased difficulty in driving at night and undertaking patient assessment and treatments are broad.
Key findings were intuitively grouped into the headings below.
Visual acuity and low luminance
VA is the measurement of the ability to resolve fine details and is typically assessed using high-contrast letter charts. It is intuitive and extensively discussed that the ability to observe fine details degrades in low light, and this is in concordance with decreasing VA in reduced light conditions (Low, 1946; Kaido et al, 2007; Kaido, 2018; Hiraoka et al, 2015).
Various factors influence low-luminance vision (Mandelbaum and Sloan, 1947; Conner, 1982; Barbur and Stockman 2010; Wilkinson et al, 2020; Wood et al, 2021a), and eyes tend to become short-sighted under low-luminance conditions (Arumi et al, 1997), with a change of approximately one dioptre reported in dark conditions as a result of night myopia (Charman, 1996).
It has also been reported that vision can become blurred in low light conditions because of spherical aberrations (Lápez-Gil et al, 2012) and poor accommodation (Leibowitz and Owens, 1975; Artal et al, 2012).
Exposure to an object or image in low light conditions for longer durations leads to improved visual acuity but not to the same level as during daylight conditions (Heinrich et al, 2020).
Driving and low luminance
Driving statistics suggest that, after alcohol, night-time driving is the leading cause of road traffic collisions (RTCs) (Williams and Preusser, 1997; Åkerstedt et al, 2001).
This is likely to be multifactorial (Owens et al, 2007) owing to factors such as age (Morgan and King, 1995; Owens et al, 2007; Shanmugaratnam et al, 2010), fatigue (Åkerstedt et al, 2001) and reduced light levels (Cohen et al, 2007; Owens et al, 2007). A decrease in night-time RTCs has been directly correlated with increased street lighting (Jackett and Frith, 2013). In addition, eye-scanning patterns and visual perceptions are altered in low light (Rackoff and Rockwell, 1975), which could impact the assessment and interpretation of hazards or the viewing of road signs.
Assessing daylight VA in isolation does not adequately determine the ability to drive safely in low light conditions (Gruber et al, 2013) and glare sensitivity along with reduced vision fields are better predictors of RTCs (Rubin et al, 2007).
It is important to consider that, because of ambient lighting and use of headlamps, driving is not undertaken in complete darkness. However, low luminance and driving while experiencing night-time myopia (>0.75 dioptres sphere myopic shift) have been attributed to a statistically significant (P=0.044) increase in night-time RTCs (Cohen et al, 2007). This suggests it may be appropriate to mandate the requirement of refractive correction for small refractive errors to optimise vision during night-time driving and improve road sign and hazard visibility.
There is a dearth of literature investigating emergency driving, vision and low luminance. The impact of blue flashing lights on driving visibility and perception as well as their impact on other road users is unknown. Accidents involving ambulances on rural roads have been associated with greater injury severity compared with those on urban roads (Weiss et al, 2001). These roads are typically poorly lit, although further investigation is needed to understand the impact of luminance alongside other contributing factors such as road conditions and road familiarity.
A study investigating the impact of luminance on driving on a circuit showed that decreasing luminance reduced the ability to recognise road signs, identify road hazards and maintain higher speeds (Wood and Owens, 2005). Identifying hazards at night that would normally be recognised almost instantaneously during the day requires longer scan times rather than changes to VA, and these longer times may contribute to this loss of ability (Rackoff and Rockwell, 1975). This is compounded by the high speeds further reducing available scan time.
Glare and its impact on driving
Glare can be defined as the loss of retinal image contrast as a result of intraocular light scatter (Aslam et al, 2007) from structures including the retina, lens and cornea (Yuan et al, 1993).
There are two key types: disability and discomfort glare. Disability glare causes a decrease in vision and can be a source of imminent danger (van den Berg, 1991; Aslam et al, 2007). Discomfort glare does not impair vision but makes viewing less comfortable. Although this does not cause danger, it can lead to long-term problems such as headaches, neck pain and eye strain, particularly with long periods of repeated exposure (van den Berg, 1991). This can have a significant impact on all tasks requiring vision as some forms of glare can be extremely disabling (Aslam et al, 2007).
There are several common ophthalmic conditions and procedures that predispose individuals to experiencing heightened glare. These include cataracts, certain types of contact lens and having undertaken refractive procedures such as LASIK (Lasa et al, 1993; Rubin et al, 1993). Therefore, this is an issue that cannot be ignored by paramedics.
Increased difficulty in driving, including in-lane control and cornering, as a result of glare, has been identified in simulated conditions; of particular concern is being less able to detect pedestrians (Ranney et al, 2000; Theeuwes et al, 2002). It has also been established that sensitivity to glare is a better predictor of RTCs than VA during photopic (daylight) conditions (Rubin et al, 1993).
Additionally, glare in bright light may result in slower driving times (Theeuwes et al, 2002), which could increase response times and therefore affect patient outcomes. Evidence to support this as a potential cause of delays to assessing or treating patients has not been identified.
Colour vision and low luminance
Colour vision is adversely affected by decreased luminance (Zele and Cao, 2015; Kelber et al, 2017) because of the shift from trichromatic, cone-mediated vision that occurs in bright light conditions to monochromatic, rod-mediated vision at low light levels.
Degradation of colour is not equal for all wavelengths. Red colours (longer wavelengths) are affected more than blue colours (shorter wavelengths) following dark adaption since spectral sensitivity between long-wavelength cones and short-wavelength rods and cones is different (Long and Garvey, 1988).
The threshold for colour vision varies substantially between colours and individuals. Yellow colours can be identified down to 10-3cd/m2 background luminance and blue colours down to 10-2cd/m2 background luminance but only when a contrasting cue is provided (Kelber et al, 2002) so this is unlikely to be a true reflection of real-world colour vision. In comparison, Roth et al (2008) reported no correct identification of colour at a luminance level of 0.007cd/m2.
Of particular note is the Purkinje shift, whereby red objects become darker while blue colours are enhanced (Figure 3) (Barlow, 1957). This may have an adverse effect on performance in emergency driving. In simulated driving conditions, it has been demonstrated that the ability to distinguish the colour red diminishes as luminance decreases (Alferdinck, 2006). This may be relevant in respect of street features such as warning signs and traffic lights. Although not evidenced in the literature found in this review, it can also be hypothesised from this that a ‘washout’ effect may be exhibited with the use of blue lights, making road features less distinguishable.
Approximately 5% of men have a red-green colour vision defect. There is no evidence that these individuals are at a disadvantage for paramedic duties as many tasks are based on comparisons between colours (Verhulst and Maes, 1998).
Difficulties in identifying skin colour changes was highlighted by the professional panel discussion as being particularly problematic. Clinically, blue skin changes observed in cyanosis are of immediate concern and needs to be identified early. The loss of ability to distinguish skin colour changes in low luminance conditions is unexplored.
Vision scientists have long been aware of the concept of colour rendering, where lamps of differently coloured hues have different effects on vision and dark adaptation levels (Verhulst and Maes, 1998). This has resulted in street lamps in many locations being changed from yellow sodium to LED lighting, which emits shades closer to natural light (Alferdinck, 2006; Masuda and Uozato, 2014). Selection of hue and luminance of any lighting devices issued needs careful consideration to enable optimal performance and patient care by paramedics.
Contrast sensitivity and low luminance
Contrast sensitivity is the lowest observable difference between an object and the background. Seeing the difference between two areas that are similar, such as the edge of the pavement against the road or skin lesions with subtle pigmentation changes, can be difficult.
The ability of the visual system to discern contrast is complex and changes with shifts in object and background luminance levels (Wood et al, 2021b). Overall, contrast sensitivity decreases with age (Gillespie-Gallery et al, 2013). For paramedics aged <40 years, the effects are unlikely to be significant in practice (Puell et al, 2004a; 2004b). However, after this age and towards the current retirement age in the UK of 66, it is plausible that reduced contrast sensitivity may affect the visibility of a range of paramedic activities.
Low-luminance contrast sensitivity changes cannot be predicted from standard VA tests. Identifying contrast sensitivity changes may be better achieved by performing targeted testing in low-luminance conditions (Ginsburg et al, 1982; Hertenstein et al, 2016). The effect on vision may be linked to the effect on response to glare rather than decline in contrast sensitivity alone (Puell et al, 2004b). Appropriate lighting and lighting set-ups can in some circumstances improve contrast sensitivity and reduce glare.
Of relevance to paramedics, work conducted in pilots shows that scotopic contrast sensitivity is a better predictor of performance on occupational tasks than standard photopic VA measures (Ginsburg et al, 1982). Using such measures in paramedic occupational assessments may allow better predictive measures.
Photopic contrast sensitivity measurements cannot predict the performance of contrast sensitivity under low light conditions (Hertenstein et al, 2016; Owsley et al, 2020) under laboratory or real-world conditions.
There is no literature investigating the relevance of visual contrast sensitivity on paramedic practice.
Individual considerations
Consideration should also be made for individual factors, such as age (Brown et al, 1987; Sturr et al, 1990; Fraade-Blanar et al, 2018) and optical health (Congdon et al, 1995; Helgesen et al, 2004; Schallhorn et al, 2009), which may influence the success and safety of tasks undertaken in low-luminance conditions.
For example, in endotracheal intubation in simulated conditions, the quality of view during direct laryngoscopy was shown to be directly affected by age-related VA changes (Mathews et al, 2021). Although this could be compensated for by changing the viewing angle and using adjuncts, its potential significance when combined with low luminance should be considered.
The rates of common conditions such as diabetes and glaucoma are increasing in the population (Flaxman et al, 2017). Even at early stages, these conditions will cause changes in visual perception which can be mitigated with some simple measures and must be accounted for to maintain a high-quality service and healthy paramedic workforce.
Meta-analysis
To understand further how VA changes with reducing luminance, a meta-analysis was conducted including seven studies that assessed VA at different luminance levels (Pesudovs et al, 2004; Barrio et al, 2015; Hiraoka et al, 2015; Lin et al, 2015; Bartholomew et al, 2016; Pluháček and Siderov, 2018; Freundlieb et al, 2020). A forest plot of mean logMAR VA reported at different luminance levels was created (Figure 4).
Three studies (Pesudovs et al, 2004; Barrio et al, 2015; Lin et al, 2015) reported a range of VAs between 0.0 and 0.5 logMAR for 0.75cd/m2, suggesting VA at this luminance level varies. VA dropped off significantly below 0.38cd/m2. This is in agreement with previous findings, where 100–1.0cd/m2 VA remains within normal limits and becomes impaired only when luminance falls below 1.0cd/m2 (Rabin, 1995).
These findings suggest that, in low-luminance conditions (<1.0cd/m2), paramedic activities requiring attention to fine details are likely to be hindered and would benefit from additional lighting to maintain good vision for such tasks.
Discussion
The main difficulties paramedics experience when working in low light conditions are in driving and patient assessment/treatment. Multiple factors affect visual abilities in low light conditions so multiple solutions are required.
The effects of low light on driving are likely to be magnified when driving an ambulance vehicle in emergency conditions where faster driving speeds must be maintained. However, the enhanced training that emergency drivers receive may offset some of this risk.
To the best of the authors' knowledge, there is no published data on the location and cause of ambulance driving incidents during night-time conditions. Analysis of such driving incidents could provide insight into risk-reducing strategies.
Glare may be of practical relevance to paramedics. Sources of glare while driving ambulance vehicles could include oncoming traffic and street lighting, reflections from flashing blue lights, illuminated satellite navigation equipment and the mobile data terminal. Dirty windscreens and a low sun position can also cause significant glare.
There are no standardised occupational health standards relating specifically to paramedics in the UK. The Health and Care Professions Council regulates allied health professionals including paramedics in the UK and standards are generic (HCPC, 2014), as is the driving standard set by the Driver and Vehicle Licensing Agency (2021) which concentrates on VA and visual fields. Neither of these consider other visual factors such as sensitivity to glare, contrast sensitivity and colour vision, which may affect driving and clinical practice. Some of these are assessed as part of occupational health checks for other professions such as pilots and train drivers. However, as far as the authors are aware, no occupational standards focus specifically on low light conditions.
In addition, assessments are generally only undertaken on initial employment and are not routinely repeated to consider changes associated with age or development of health conditions. This review has identified that many aspects of visual function, including colour vision and contrast sensitivity, are impacted by low luminance, along with environmental factors such as glare. Current assessments are insufficient to fully assess the effectiveness and safety of paramedic practice in low lighting conditions.
Recommendations
To overcome the difficulties identified in this review, the authors recommend that national paramedic occupational health standards be developed for low light working conditions with visual function assessments established to ensure adherence to these. Assessments should be designed to simulate realistic working conditions by incorporating testing in a variety of lighting conditions as well as assessing response to glare.
A collaborative working group between optometry and ambulance trusts and clinicians is required to develop an array of tests to probe visual function to provide evidence-based support for the workforce. In recognition that visual function may change throughout a paramedic's career, especially as age increases, a full range of assessments that examine visual function in a range of lighting conditions should be undertaken at appropriate intervals.
Occupational support should be tailored to individual paramedics and conducted supportively to ensure an open culture where self-referral is encouraged when there are changes in personal health. To avoid concern and distress, a sensitive approach is needed with the underlying aim of retaining and supporting experienced paramedics to work in the prehospital environment for as long as they wish to and are able to do so safely. To support paramedics, a package of evidence-based adjustments should be made available. In exceptional circumstances, this could include duties that avoid driving at night. Adaptations to working practices should be practical and cost-efficient.
Adjustments may be needed for all paramedics to ensure safety when working in low light conditions. In the UK, there are no restrictions on the speed of emergency response during night-time working (Association of Ambulance Chief Executives, 2018) and no distinctions made between luminance levels. While evidence is limited and further research is required to confirm whether a higher risk of night-time RTCs may be linked to reduced VA, driving at slower speeds may be necessary for safety to be maintained. This could potentially increase response times and, ultimately, affect the outcomes of critically unwell patients; careful consideration for resource planning would therefore be needed if guidelines are amended. Excessive glare from vehicle-based or portable equipment should be considered before implementation and may be corrected with low-cost filters and software changes.
Adjustments to the working environment of paramedics could reduce the difficulties experienced when treating patients in low light conditions and, potentially, lead to better clinical outcomes. All panel respondents said that equipment to increase scene luminance would improve their ability to treat patients. Therefore, a review of the lighting equipment included within the ambulance inventory should be undertaken in conjunction with lighting engineers to determine whether additional equipment would be beneficial.
Whether this is vehicle-based, such as fixed or portable flood lighting, or personally-issued hand and head torches, lighting enhancements should be informed with the involvement of all stakeholders. Consideration should be given to the risk of exacerbating some of the problems experienced such as excessive glare, over illuminating the scene and losing contrast. This is especially important for activities requiring assessment of skin colour.
Limitations
The sample size of the professional panel was small and, although this had not been the intention, all worked for the same ambulance service NHS trust so may not be representative of the full range of paramedics working in the UK or internationally. The respondents shared similar experiences but it cannot be inferred that these experiences are universal.
The scoping review revealed limited research into evidence-based occupational standards and associated visual function in paramedic practice. Therefore, the literature search was expanded to include visual function scenarios highlighted by the professional panel that could be extrapolated to paramedic practice.
As a result, the authors can make only limited conclusions and recommendations for further investigation more tailored to paramedic practice.
Although this study has focused on paramedic practice, many of the difficulties presented with difficult working conditions in low light extend to other emergency services such as the fire and police services, as well as to work undertaken in war zones. While these were beyond the scope of this article, they also deserve attention.
Conclusions
This study is the first to explore the experiences of paramedics working in low-luminance conditions. Visually demanding tasks are hindered by low light conditions. Despite broad and frequent low luminance working conditions in paramedic practice, there is a lack of evidence to inform occupational requirements for low light working. More research is needed to create occupational standards and inform personal occupational assessments to ensure that paramedics are supported to undertake their role safely and effectively so they can enable optimal patient care. These findings could be relevant to other emergency service workers.