The association between smoking and marijuana is often considered as an interaction effect for pregnancy complications, as the majority of women who use marijuana also smoke cigarettes. In fact, amongst women who used marijuana in the SCOPE cohort, 74% also smoked cigarettes. With a high concurrence rate, the independent effect of marijuana on pregnancy outcomes has generally been unrecognised and just considered to be subsidiary, partly due to the low availability of data on marijuana use compared to cigarette smoking for statistical analysis. However, our data from the SCOPE cohort, with 316 participants who were marijuana users, demonstrate that the association of marijuana use with SPTB is consistent across cigarette smokers and non-smokers. The consistent effect of marijuana use is also apparent when analysing the effect of the estimated number of episodes of marijuana use during pregnancy on the length of gestation. While there was a slight decrease in the predicted length of gestation amongst smokers, the trend for smokers and non-smokers was similar. In contrast, the predicted length of gestation for women who continued to use marijuana at 20 weeks’ gestation was significantly decreased compared to those who ceased earlier in gestation, regardless of smoking status. This is consistent with similar studies which showed that marijuana use is associated with a decreased length of gestation. Furthermore, apart from a cigarette smoking-marijuana interaction, it is also well recognised that cigarette smoking and illicit drug use are associated with low socio-economic status,vertical grow system along with a complex inter-relationship with obesity, where smoking cessation may also lead to obesity.

As described in many studies, the prevalence of cigarette smoking and obesity is higher amongst those who are socio-economically disadvantaged, and the incidence of SPTB is higher amongst women with lower income and lower educational status, which may indicate associations with other lifestyle risk factors. Furthermore, if there was no maternal marijuana exposure, with an estimated population attributable risk of 0.003 for marijuana use, the incidence of SPTB would be expected to decrease by 3 cases per 1000 pregnant women. With an overall rate of SPTB of 4.2% in this study, this represents an estimated 6.2% reduction in the incidence of SPTB in the population, i.e. about 3 out of 50 SPTB cases would be attributed to marijuana use. If we consider the Australian centre only, where any marijuana usage occurred in 11.6% of women compared to 3.6–4.5% in the other centres, the estimated PAR was 0.009 for marijuana use with an expected reduction of SPTB of 9 cases per 1000 pregnant women, and a 11.68% reduction in the incidence of SPTB in this centre if women did not use marijuana. That is, in the Australian study centre, almost 12% of SPTB could be attributable to maternal marijuana use.A major strength of this study was its large international multicentre prospective cohort with excellent follow-up and complete data available for this analysis. Women were recruited from a clearly defined population of nulliparous women, with meticulous data monitoring protocols to reduce data entry or transcription errors and ensure the quality of data. While there are other studies that have examined the effect of marijuana use on adverse pregnancy outcomes, interaction tests were not performed. Hence, with complete quality data available from this study, interactions between marijuana use and cigarette smoking status may be examined while also adjusting for potential confounders. It needs to be noted that the number of SPTB cases amongst women who reported marijuana use at 20 weeks’ gestation is small even in this large cohort. The use of self-reported marijuana use and cigarette smoking status may be a potential limitation, as it may be subject to participant recall bias. Furthermore, this study was undertaken in a nulliparous cohort so it may be the case that our findings apply only to nulliparous women.

Although medication for maternal asthma, thyroid disease, and PCOS were recorded, we found no evidence of association with pregnancy outcomes analysed in this study, therefore these were not included in the analysis. Further research is required to confirm these findings, and future studies should include appropriate corrections for the various important confounders.Bio-aggregate composites are formed from a mineral binder and bio-aggregate particles such as hemp, sun flower or rape stalk. The wet mixture can be cast or projection formed around a structural frame as an insulating infill with such walls exhibiting lower embodied energy than traditional alternatives. In addition composites of hemp and lime have been shown to offer a beneficial compromise between thermal conductivity and thermal inertia, enabling the passive moderation of building climate. Despite these advantages the application and utilisation of bio-aggregate composites remains low in construction due in part to the high variability of a bio-sourced product and in part to the conservative nature of the industry. A better understanding of the material’s performance, leading to control and optimisation of physical properties, is a necessary step in addressing these issues. As composite materials, alterations to the ratio and nature of the constituents are logical avenues of material development. The ratio of binder to aggregate and its influence on thermal conductivity and mechanical strength of hemp-lime have been studied extensively with unanimous agreement that an increase in the binder quantity increases the compressive strength, flexural strength and thermal conductivity. This is attributed to an increasing binder content tending the behaviour of the composite to that of the binder and several models for thermal conductivity and compressive strength have been proposed based broadly on this assessment. Mechanical properties and thermal conductivity have been modelled simplistically as a function of density, considered an indicator of binder content, and more recently, through multi-phase models that specifically accounts for the ratio of constituents.

While such models are logical, it should be noted that they assume an isotropic relationship in a material that is known to have an anisotropic structure and behaviour.While in all studies reviewed, particle size distribution has been shown to impact on the physical properties of materials, although there has been limited consensus between studies as to the relationship. It has been observed by some that a finer grade of particles provides an increase in mechanical strength and a smaller increase in thermal conductivity, attributable to a closer packing of the particles. Contrastingly, others found that a coarser grade of particles yielded better mechanical properties, attributed to a greater overlap of particle. A possible reason for the disagreement of these conclusions may be the small fluctuations in properties reported compared to relatively large natural variation. Alternatively it may also be argued that previous studies tend to express bio-aggregate grades simply as finer or coarser based on average length; other potentially significant factors such as the spread of the distribution or aspect ratio of particles may thus have been overlooked. Within the body of previous work it is noticeable that the effect of changing these variables is often only reported in one orientation but the observed relationships are assumed to apply globally. It is now known that the internal structure of the bio-aggregate composite hemp-lime is orientated as a result of the production method chosen and the elongated form of the particles. It has also been identified from a number of sources that the mechanical behaviour and thermal conductivity of bio-aggregate composites are anisotropic that may be attributed to the structure and presumed to apply in all cases where the bio-aggregates are elongated. It is therefore necessary to consider any influence of constituent variables within this context meaning it cannot be assumed that constituent variables have an isotropic effect. To the author’s knowledge it has not previously been ascertained if changing the binder concentration or the particle size distribution has a global or directionally dependant influence on physical properties.

This paper considers the thermal conductivity,cannabis grow equipment compressive strength and flexural strength of hemp-lime specimens produced with three ratios of hemp to binder and three distinct grades of hemp aggregate; the particle distribution of the hemp aggregate fully characterised in each instance by means of two dimensional imaging. In order to ascertain if any effects are directionally dependent, thermal and mechanical tests were conducted in two directions: parallel to the direction of casting force and perpendicular to it. A recently developed method for assessment of the internal structure of bio-aggregate composites was also used in each case to provide an insight into the internal topology and to help inform any conclusions drawn about the mechanisms involved.Five mixtures of hemp-lime were considered in the study covering a range of three distinct grades of hemp aggregate and three hemp to binder ratios.Hemp lime was chosen due to it being the most prominently assessed bio-aggregate composite within the literature and indeed industry. In order to minimise the total amount of material used, single sets of rectangular prism specimens were produced for all mechanical tests as well as the internal structure assessment.In all cases two sets of specimens were cast, one for testing parallel to the casting compaction and one perpendicular. The specimens were produced by first combining water and the binder in a revolving pan mixer to produce a uniform slurry. Once uniform, the hemp aggregate was added and further mixing conducted until a homogenous mixture was observed. The total mixing time was under 5 min in each case. The binder used throughout the study was a commercially available preformulated binder for use with bio-aggregates produced by Tradical. The hemp aggregate used was grown and processed in France and supplied by the producer in four grades 7, 8, 12 and 14. The three grades used for this study were 7 , 14 and 1:1 by mass mixture of 8 and 12. The rationale of mixing two of the manufactures’ grades to produce the medium grade was to ensure a wider distribution of particle sizes in this grade compared to the fine and coarse grades. The combined mixture was weighed out into the moulds prepared with release oil in 50 mm layers with light tamping between each layer. The amount of material weighed out in each case was predetermined in order to produce a similar compaction state across the specimens. In this study this was set at 45% densification of loose-state density, determined for each mix by weighing a set volume of un-compacted material placed carefully by hand,. The specimens were stored after production in a conditioned room at 20  C and 70% relative humidity, uncovered in the moulds for 6 days and uncovered out of the moulds thereafter.The three grades of aggregate used in this study: fine, medium and coarse were assessed for particle size distribution by a method of two dimensional image analysis developed by Picandet. This was selected over a simple sieving method in order to provide data about both particle length and width.

The analysis was conducted on a 20 g sample of each grade removed from a 20 kg bag by a process of quartering. Scanning was conducted by arranging a small amount of particles with their largest surface face down on the surface of a flatbed scanner by hand in order to segregate them. This was then scanned against a blue background at a resolution of 1200 dpi and the process repeated until the full 20 g was imaged. All image processing and measurements were conducted using the program ImageJ and the method used follows that described by Picandet and reported here in brief. In each case a colour threshold was applied to the image to produce a binary image of the hemp. The binary images were then enhanced using three iterations of an opening algorithm to remove noise and dust. Assessment of each image was conducted using the particle analysis tool that identifies the primary and secondary axis of each discrete binary object. This is done though equalising the particle’s second moments of area to that of an ellipse whose axes are then used to provide a measure of length and width. To produce a distribution comparable to a sieving analysis, an estimated volume for each particle is calculated based on the area of the particles and an assumption that average thickness is proportional to particle width.